drm/amd/display: Allow idle optimization based on vblank.

[Why]
idle optimization was being disabled after commit.

[How]
check vblank count for display off and enable idle optimization based on this count.
Also,check added to ensure vblank count does not decrement, when count reaches 0.

Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>