drm/amdgpu: make amdgpu_to_sched_priority detect invalid parameters

Returning invalid priorities as _NORMAL is a backwards compatibility
quirk of amdgpu_ctx_ioctl(). Move this detail one layer up where it
belongs.

Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>