drm/amd/display: use max_dsc_bpp in amdgpu_dm

Since, the quirk is handled in the DRM core now, we can use that value
instead of the internal value.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>