drm/imagination: vm: fix drm_gpuvm reference count

The driver specific reference count indicates whether the VM should be
teared down, whereas GPUVM's reference count indicates whether the VM
structure can finally be freed.

Hence, free the VM structure in pvr_gpuvm_free() and drop the last
GPUVM reference after tearing down the VM. Generally, this prevents
lifetime issues such as the VM being freed as long as drm_gpuvm_bo
structures still hold references to the VM.

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/20231124233650.152653-4-dakr@redhat.com