gpu/drm: Eliminate DRM_SCHED_PRIORITY_UNSET

Eliminate DRM_SCHED_PRIORITY_UNSET, value of -2, whose only user was
amdgpu. Furthermore, eliminate an index bug, in that when amdgpu boots, it
calls drm_sched_entity_init() with DRM_SCHED_PRIORITY_UNSET, which uses it to
index sched->sched_rq[].

Cc: Alex Deucher <Alexander.Deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Alex Deucher <Alexander.Deucher@amd.com>
Link: https://lore.kernel.org/r/20231017035656.8211-2-luben.tuikov@amd.com