drm/amd/display: Drop DMCUB from DCN21 resources

The interface to the DMCUB won't be through DC itself. DC will instead
call into the DMUB interface introduced with a future change.

The CONFIG_DRM_AMD_DC_DMUB defines will still be used for now but will
be dropped at the end of the series.

Since this define was never configurable in the first place this code
wasn't used.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>