drm/amd/display: Run idle optimizations at end of vblank handler

[Why & How]
1. After allowing idle optimizations, hw programming is disallowed.
2. Before hw programming, we need to disallow idle optimizations.

Otherwise, in scenario 1, we will immediately kick hw out of idle
optimizations with register access.

Scenario 2 is less of a concern, since any register access will kick
hw out of idle optimizations. But we'll do it early for correctness.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>