drm/amd/display: Remove unused P010 debug flag

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Mike Hsieh <chun-wei.hsieh@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>