drm/amd/display: Clean up flattening core_dc to dc

Clean up some code related to flattening core_dc commit
(Remove redundent dc = dc, which was the result of removing
DC_TO_CORE() macro)

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>