drm/amd/display: Add the DMUB service

The DMUB service is the interface to the DMCUB.

It's required to support Renoir features so it will be enabled and
compiled automatically when the Renoir display engine is enabled via
CONFIG_DRM_AMD_DC_DCN2_1.

DMUB code will initially be guarded by CONFIG_DRM_AMD_DC_DMUB and later
switched to CONFIG_DRM_AMD_DC_DCN2_1 with the config option dropped.

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>