drm/amdgpu: allow for more flexible priority handling

Allow to call amdgpu_ring_priority_get() after pushing the ring to the
scheduler.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>