[REQUEST] Add support for Intel DPST (Display Power Saving Technology)

[José Relvas]

Hey folks,

I've noticed that power-profiles-daemon recently added support for AMD's panel power saving technology with this MR:
https://gitlab.freedesktop.org/upower/power-profiles-daemon/-/merge_requests/137

It appears to be controlled via a panel_power_saving SYSFS property.

This reminded that Intel has a similar technology. RPL's datasheets confirm this:
https://edc.intel.com/content/www/us/en/design/products/platforms/details/raptor-lake-s/13th-generation-core-processors-datasheet-volume-1-of-2/display-power-savings-technologies/

> The Intel® DPST technique achieves back-light power savings while maintaining
> a good visual experience. This is accomplished by adaptively enhancing the
> displayed image while decreasing the back-light brightness simultaneously. 
> The goal of this technique is to provide equivalent end-user-perceived 
> image quality at a decreased back-light power level. 

> Intel® OPST solution uses same HW infrastructure as Intel® DPST. Frames are 
> processed using frame change threshold based interrupt mechanism similar
> to Intel® DPST. Intel® OPST SW algorithm determines which pixels in the 
> frame should be dimmed to save power keeping visual quality (such as contrast, color) 
> impact to acceptable level. Since there is no backlight for OLED panels,
> the power savings come solely from pixel dimming. 

However, it doesn't seem like i915 has any support for this. Searching online was
ineffective too:

- I found mentions of /sys/class/drm/card0/power/i915_dpst, but it doesn't seem to exist,
at least not anymore.

- A i915.dpst parameter was also brought up, but it doesn't seem to exist either.

Interestingly, all mentions of dpst on Linux were referring to Android, so maybe
this was a downstream thing with Android?

In any case, I found this email in the archive which confirms
that DPST was not supported (at least in 2012):
https://lore.kernel.org/intel-gfx/50ADFFB3.8030907@linux.intel.com/

Since AMD's equivalent to DPST is now supported by their driver and being used by userspace,
it seems like a sensible idea to implement this for i915. I've confirmed that this feature
has existed since (at least) Haswell (released over a decade ago!) so implementing it
would likely net a few energy consumption improvements for a large chunk of Intel laptops.

The best approach here would probably be to expose a similar attribute to amdgpu's 
"panel_power_savings", with a scale that controls the feature's aggressiveness,
then update userspace tools, including power-profiles-daemon, to set the value
based on the intended energy scheme.

Thanks for reading,
José Relvas