Enable vblank_disable_immediate on more drivers.

[Mario Kleiner]

This patch series sets dev->vblank_disable_immediate = true on
radeon/amdgpu-kms, nouveau-kms for nv50+, and vc4 for the real
kms driver (as opposed to dispmanx firmware backed kms).

All the drivers should be ready in theory, given their implementation,
for fast vblank disable/enable. In practice, i have performed timing
tests with my measurement equipment for all those drivers with the
current 4.12.0-rc5 based drm-next tree, and the rpi-4.12.0-rc5
Raspberry Pi kernel tree under Raspian during the last days, and
everything seems to work fine.

Thanks to Chris Wilsons nice vblank on/off/query optimizations from
Linux 4.12 this should now generally be a win for power saving without
degradation in performance, so time to flip the switch on and see if
it works out on non-Intel gpu's as well.

One restriction for vc4: I couldn't test yet if this could cause
any trouble for the vc4-fkms-v3d devicetree binding, using the firmware
based kms path. For some reason the rpi-4.12-rc5 kernel never makes it
to a working display if i choose vc4-fkms-v3d instead of vc4-kms-v3d dt.
This also happens with the unpatched kernel, so is not related to this
patch, but with a black screen, dead network and nothing logged in any log,
i gave up debugging this atm. Just to say, that path is untested, only
the proper kms driver is successfully tested.

thanks,
-mario