drm/panthor: Don't call drm_gpuvm_bo_extobj_add() if the object is private

drm/panthor: Don't call drm_gpuvm_bo_extobj_add() if the object is private

drm_gpuvm_bo_extobj_add() is a NOP if the object is private, but it
forces us to take/release the VM resv lock, so let's do that only when
we know the object can be shared.

v3:
- New commit

v4:
- Collect R-bs

v5:
- No changes

v6:
- No changes

v7:
- No changes

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patch.msgid.link/20260401134854.2275433-6-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>