Enable fp16 display support for DCE8+, next try.

[Mario Kleiner]

Hi and happy post-christmas!

I wrote a patch 1/1 that now checks plane scaling factors against
the pixel-format specific limits in the asic specific dc_plane_cap
structures during atomic check and other appropriate places.

This should prevent things like asking for scaling on fp16 framebuffers
if the hw can't do that. Hopefully this will now allow to safely enable
fp16 scanout also on older asic's like DCE-11.0, DCE-10 and DCE-8.
Patch 2/2 enables those DCE's now for fp16.

I used some quickly hacked up of IGT test kms_plane_scaling, manually
hacking the src fb size to make sure the patch correctly accepts or
rejects atomic commits based on allowable scaling factors for rgbx/a
8 bit, 10, and fp16.

This fp16 support has been successfully tested with a Sea Islands /
DCE-8 laptop. I also confirmed that at least basic HDR signalling
over HDMI works for that DCE-8 machine with a HDR monitor. For this
i used the amdvlk driver which exposes fp16 since a while on supported
hw.

There are other bugs in DC wrt. DCE-8 though, which didn't prevent
my testing, but may be worth looking into. My DCE-8 machine scrambles
the video output picture somewhat under Vulkan (radv and admvlk) if the
output signal precision isn't 8 bpc, ie. on 6 bpc (eDP laptop panel)
and 10 bpc, 12 bpc (HDMI deep color on external HDR monitor).

Another fun thing is getting a black screen if DC is enabled on at least
Linux 5.10+ (but not if i use the classic kms code in amdgpu-kms). If
i recompile the driver with a Ubuntu kconfig for Linux 5.9, the 5.10
kernel works, and the only obvious DC related difference is that DC's
new SI / DCE-6 asic support is disabled at compile time.

Thanks,
-mario