drm/imagination: vm: prevent duplicate drm_gpuvm_bo instances

Use drm_gpuvm_bo_obtain() instead of drm_gpuvm_bo_create(). The latter
should only be used in conjunction with drm_gpuvm_bo_obtain_prealloc().

drm_gpuvm_bo_obtain() re-uses existing instances of a given VM and BO
combination, whereas drm_gpuvm_bo_create() would always create a new
instance of struct drm_gpuvm_bo and hence leave us with duplicates.

Fixes: ff5f643d ("drm/imagination: Add GEM and VM related code")
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Donald Robson <donald.robson@imgtec.com>
Tested-by: Frank Binns <frank.binns@imgtec.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231124233650.152653-2-dakr@redhat.com