drm/amd/display: Add debug option for idle reg checks

[Why]
Leave disabled by default due to sequencing issues around power states
where these flags aren't properly reset.

[How]
Allow re-enabling from DC debug option.

Reviewed-by: Gabe Teeger <gabe.teeger@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>