drm/amdgpu: fix num_ibs check

Specifying no IBs on command submission is invalid, stop crashing
badly when somebody tries it.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucer@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>