drm/amd/display: Prevent vblank irq disable while VRR is active. (v3)

drm/amd/display: Prevent vblank irq disable while VRR is active. (v3)

During VRR mode we can not allow vblank irq dis-/enable
transitions, as an enable after a disable can happen at
an arbitrary time during the video refresh cycle, e.g.,
with a high likelyhood inside vblank front-porch. An
enable during front-porch would cause vblank timestamp
updates/calculations which are completely bogus, given
the code can't know when the vblank will end as long
as we are in front-porch with no page flip completed.

Hold a permanent vblank reference on the crtc while
in active VRR mode to prevent a vblank disable, and
drop the reference again when switching back to fixed
refresh rate non-VRR mode.

v2: Make sure transition is also handled if vrr is
    disabled and stream gets disabled in the same
    atomic commit by moving the call to the transition
    function outside of plane commit.
    Suggested by Nicholas.

v3: Trivial rebase onto previous patch.

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>