drm/i915: Truncate the shmem backing pages on purge

shmfs doesn't actually implement i_ops->truncate() so we were not
immedatiately releasing the backing pages when shrinking the gfx cache
under OOM. Instead use a combination of truncate_inode_pages() and
i_ops->truncate_range() as is used by shmem_delete_inode().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>