drm/etnaviv: Remove duplicate call to drm_gem_free_mmap_offset

The docs explicitly say the drm_gem_object_release function already calls this,
and this does not appear to be a prerequisite for the call to
etnaviv_gem_ops.release.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>