drm/amdgpu: introduce AMDGPU_CTX_PRIORITY_UNSET

Use _INVALID to identify bad parameters and _UNSET to represent the
lack of interest in a specific value.

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>