drm/amd/display: Don't consider vblank passed if currently in vertical front porch time

Changing refresh rates on OLED displays works differently to typical
LCD panels in that instead of changing the clock, the vertical porch
is extended significantly for lower rates.

This can mean that the vertical porch can be incredibly large for
non-base refresh rates eg. 60Hz on a 90Hz display.

This isn't an issue for X11/typical compositors as their present slop
is 1/2th of the refresh interval so the issue never manifests.

However in Gamescope, the present slop very small and tuned to be
optimal in real-time to try and reduce display latency significantly.
This results in us queueing up the atomic commit inside the vertical
porch region which, due to legacy X11/sync control reasons, means that
AMDGPU must target the next vblank.

This patch changes that behaviour to make FRR displays match what occurs
on VRR/Freesync displays where the vertical porch time is not included
in determining what vblank to target and solves the issue.

This means that smarter compositors can get large input latency
reductions when using OLED displays at lower than base refresh rates.

For upstreaming this patch, it will need to be considered what the best
solution is to enable this behaviour from the userspace side.
Obviously the X11/legacy stuff probably cannot change here -- so we
either need to enable this new behaviour globally for all DRM atomic
clients (ie. basically Wayland compositors) or have a
new DRM_MODE_ATOMIC flag.

Signed-off-by: Joshua Ashton <joshua@froggi.es>

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