drm/amdgpu: lower BUG_ON into WARN_ON for AMDGPU_PL_PREEMPT

That should never happen, but make sure that we only warn instead of
crash.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>