drm/amd/display: Guard cursor idle reallow by DC debug option

drm/amd/display: Guard cursor idle reallow by DC debug option

[WHY]
To control whether idle optimizations reallowed after the first cursor
update.

[HOW]
Add checks to the conditions.

Reviewed-by: Duncan Ma <duncan.ma@amd.com>
Acked-by: Alex Hung <alex.hung@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>