drm/imagination: Free pvr_vm_gpuva after unlink

drm/imagination: Free pvr_vm_gpuva after unlink

This caused a measurable memory leak. Although the individual
allocations are small, the leaks occurs in a high-usage codepath
(remapping or unmapping device memory) so they add up quickly.

Fixes: ff5f643de0bf ("drm/imagination: Add GEM and VM related code")
Cc: stable@vger.kernel.org
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Link: https://patchwork.freedesktop.org/patch/msgid/35867394-d8ce-4698-a8fd-919a018f1583@imgtec.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>