drm/shmem-helper: Add .gem_create_object helper that sets map_cached flag

The helper drm_gem_shmem_create_object_cached() allocates an GEM SHMEM
object and sets the map_cached flag. Useful for drivers that want cached
mappings.

v3:
	* style fixes

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200609090820.20256-2-tzimmermann@suse.de