drm/amd/display: Compensate for pre-DCE12 BTR-VRR hw limitations. (v3)

drm/amd/display: Compensate for pre-DCE12 BTR-VRR hw limitations. (v3)

Pre-DCE12 needs special treatment for BTR / low framerate
compensation for more stable behaviour:

According to comments in the code and some testing on DCE-8
and DCE-11, DCE-11 and earlier only apply VTOTAL_MIN/MAX
programming with a lag of one frame, so the special BTR hw
programming for intermediate fixed duration frames must be
done inside the current frame at flip submission in atomic
commit tail, ie. one vblank earlier, and the fixed refresh
intermediate frame mode must be also terminated one vblank
earlier on pre-DCE12 display engines.

To achieve proper termination on < DCE-12 shift the point
when the switch-back from fixed vblank duration to variable
vblank duration happens from the start of VBLANK (vblank irq,
as done on DCE-12+) to back-porch or end of VBLANK (handled
by vupdate irq handler). We must leave the switch-back code
inside VBLANK irq for DCE12+, as before.

Doing this, we get much better behaviour of BTR for up-sweeps,
ie. going from short to long frame durations (~high to low fps)
and for constant framerate flips, as tested on DCE-8 and
DCE-11. Behaviour is still not quite as good as on DCN-1
though.

On down-sweeps, going from long to short frame durations
(low fps to high fps) < DCE-12 is a little bit improved,
although by far not as much as for up-sweeps and constant
fps.

v2: Fix some wrong locking, as pointed out by Nicholas.
v3: Simplify if-condition in vupdate-irq - nit by Nicholas.
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>