drm/scheduler: Remove priority macro INVALID (v2)

Remove DRM_SCHED_PRIORITY_INVALID. We no longer
carry around an invalid priority and cut it off
at the source.

Backwards compatibility behaviour of AMDGPU CTX
IOCTL passing in garbage for context priority
from user space and then mapping that to
DRM_SCHED_PRIORITY_NORMAL is preserved.

v2: Revert "res"  --> "r" and
           "prio" --> "priority".

Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>