drm/amd/display: temp w/a for dGPU to enter idle optimizations

[Why&How]
vblank immediate disable currently does not work for all asics. On
DCN401, the vblank interrupts never stop coming, and hence we never
get a chance to trigger idle optimizations.

Add a workaround to enable immediate disable only on APUs for now. This
adds a 2-frame delay for triggering idle optimization, which is a
negligible overhead.

Fixes: 58a261bf ("drm/amd/display: use a more lax vblank enable policy for older ASICs")
Fixes: e45b6716 ("drm/amd/display: use a more lax vblank enable policy for DCN35+")
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>