drm/amdgpu/gfx12: return early in preempt_ib()

When MES is enabled KIQ is not available.  Return an error
when someone uses the debugfs preempt test interface in
that case.

Acked-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>