drm/amd/display: setup system context for APUs

Scatter/gather is APU feature starting from carrizo.
adev->apu_flags is not used for all APUs.
adev->flags & AMD_IS_APU can be used for all APUs.

Signed-off-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org