drm/imagination: take paired job reference

For paired jobs, have the fragment job take a reference on the
geometry job, so that the geometry job cannot be freed until
the fragment job has finished with it.

The geometry job structure is accessed when the fragment job is being
prepared by the GPU scheduler. Taking the reference prevents the
geometry job being freed until the fragment job no longer requires it.

Fixes a use after free bug detected by KASAN:

[  124.256386] BUG: KASAN: slab-use-after-free in pvr_queue_prepare_job+0x108/0x868 [powervr]
[  124.264893] Read of size 1 at addr ffff0000084cb960 by task kworker/u16:4/63

Cc: stable@vger.kernel.org
Fixes: eaf01ee5 ("drm/imagination: Implement job submission and scheduling")
Signed-off-by: Brendan King <brendan.king@imgtec.com>
Reviewed-by: Matt Coster <matt.coster@imgtec.com>
Link: https://lore.kernel.org/r/20250318-ddkopsrc-1337-use-after-free-in-pvr_queue_prepare_job-v1-1-80fb30d044a6@imgtec.com


Signed-off-by: Matt Coster <matt.coster@imgtec.com>