drm/amd/display: enable idle optimizations for linux (MALL stutter)

[Why]
We can only use this feature when the display is idle. When active vblank
irq count is 0 we know all the displays are idle.

[How]
-Add a active vblank irq counter
-Update the counter when we enable/disable vblank irq
-if vblank irq count is 0 we can consider mall stutter

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Nick Kazlauskas <Nicholas.Kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>