drm: Fix an error pointer dereference()

The drm_mode_create_tile_group() is only called from
drm_parse_tiled_block() and the caller expects it to return a NULL on
error.  In other words, this function should match
drm_mode_get_tile_group().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181217065733.GA12159@kadam