amdgpu/cs: split out fence dependency checking (v2)

This just splits out the fence depenency checking into it's
own function to make it easier to add semaphore dependencies.

v2: rebase onto other changes.

v1-Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>