drm/imagination: vm: make use of GPUVM's drm_exec helper

Make use of GPUVM's drm_exec helper functions preventing direct access
to GPUVM internal data structures, such as the external object list.

This is especially important to ensure following the locking rules
around the GPUVM external object list.

Fixes: ff5f643d ("drm/imagination: Add GEM and VM related code")
Reviewed-by: Donald Robson <donald.robson@imgtec.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231129220835.297885-3-dakr@redhat.com