drm/shmem-helper: Removed drm_gem_shmem_create_object_cached()

Cached page mappings are now the default for SHMEM GEM objects. Remove
the obsolete create function for cached mappings.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117133156.26822-3-tzimmermann@suse.de