drm/amdgpu: do not set nil entry in compute_prio_sched

If there are no high priority compute queues available then set normal
priority sched array to compute_prio_sched[AMDGPU_GFX_PIPE_PRIO_HIGH]

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>