drm/imagination: Fix reference and vm_bo handling in remap()

When a map overlaps part of an existing mapping, pvr_vm_gpuva_remap()
splits the mapping into prev/next parts covering what the request did
not take, instead of creating something new. It gets two things wrong.

- A GEM reference is taken for each part but it's never needed and never
  dropped, so it leaks one reference per split (remap-next-in-2m in
  tests/imagination/pvr_vm_map.c):

    CRITICAL: 33558528 bytes of shmem still held after close

- The parts still belong to the original object being split, but
  pvr_vm_gpuva_remap() links them to ctx->gpuvm_bo, the new object's
  vm_bo:

    prev_va --obj--> BO_A          BO_B <--obj-- vm_bo (ctx->gpuvm_bo)
            \___________link___________/  (mismatch: BO_A != BO_B)

  drm_gpuva_link() catches the mismatch:

    WARNING: drivers/gpu/drm/drm_gpuvm.c:2108 at drm_gpuva_link+0x2ec/0x310
      drm_WARN_ON(obj != vm_bo->obj)
    Call trace:
     drm_gpuva_link
     pvr_vm_gpuva_remap
     __drm_gpuvm_sm_map
     pvr_vm_map
     pvr_ioctl_vm_map

  Link them to op->remap.unmap->va->vm_bo instead.

The locking of the GPUVA lists of the other objects touched by a split is
not addressed here; it is handled by switching the GPUVM to immediate
mode.

Fixes: ff5f643de0bf ("drm/imagination: Add GEM and VM related code")
Signed-off-by: Gyeyoung Baek <gye976@gmail.com>
Reviewed-by: Brajesh Gupta <brajesh.gupta@imgtec.com>
Link: https://patch.msgid.link/20261001-pvr-fixes-a-v2-1-f58254e5dfb8@gmail.com
Signed-off-by: Alessio Belle <alessio.belle@imgtec.com>