drm/imagination: Rename pvr_queue_fence_is_ufo_backed() to reflect usage

drm/imagination: Rename pvr_queue_fence_is_ufo_backed() to reflect usage

This function is only used by the synchronization code to figure out if
a fence belongs to this driver.
Rename it to pvr_queue_fence_is_native() and update its documentation to
reflect its current purpose.

Signed-off-by: Alessio Belle <alessio.belle@imgtec.com>
Reviewed-by: Brajesh Gupta <brajesh.gupta@imgtec.com>
Link: https://patch.msgid.link/20260330-job-submission-fixes-cleanup-v1-4-7de8c09cef8c@imgtec.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>