drm/amd/display: setup system context in dm_init

[why]
display S/G mode fails in Renoir

[how]
Setup system context in dm init.

Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>