drm/panthor: Avoid adding of kernel BOs to extobj list

The kernel BOs unnecessarily got added to the external objects list
of drm_gpuvm, when mapping to GPU, which would have resulted in few
extra CPU cycles being spent at the time of job submission as
drm_exec_until_all_locked() loop iterates over all external objects.

Kernel BOs are private to a VM and so they share the dma_resv object of
the dummy GEM object created for a VM. Use of DRM_EXEC_IGNORE_DUPLICATES
flag ensured the recursive locking of the dummy GEM object was ignored.
Also no extra space got allocated to add fences to the dma_resv object
of dummy GEM object. So no other impact apart from few extra CPU cycles.

This commit sets the pointer to dma_resv object of GEM object of
kernel BOs before they are mapped to GPU, to prevent them from
being added to external objects list.

v2: Add R-bs and fixes tags

Fixes: 8a1cc07578bf ("drm/panthor: Add GEM logical block")
Signed-off-by: Akash Goel <akash.goel@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251120172118.2741724-1-akash.goel@arm.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>