staging: etnaviv: fix get_pages() failure path

get_pages() tries to convert a page array into a scatterlist.  If this
fails, we bail out without freeing the page array.  Add a call to
drm_gem_put_pages() to drop the reference gained in drm_gem_get_pages(),
indicating that we didn't access the pages.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>