drm/shmem: Put pages independent of a SG table being set

If a driver does its own management of pages, the shmem helper object's
pages array could be allocated when a SG table is not. There's not
really any  good reason to tie putting pages with having a SG table when
freeing the object, so just put pages if the pages array is populated.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Rob Herring <robh@kernel.org>