drm/amd/display: setup system context for APUs

[Why]
Scatter/gather is APU feature.
But in dm it is limited only to Renoir.
Now we need it for Vangogh.

[How]
Apply system context setup in dm_init to all APUs.

Signed-off-by: Roman Li <roman.li@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>