drm/amdgpu: make set_prt callback optional and fix error handling

PRT support is completely implemented now and we left it
turned on accidentially in the error path.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>