drm/udl: Remove udl implementation of GEM's free_object()

Udl's custom implementation for struct drm_gem_object_funcs.free_object
unmaps perma-mapped memory buffer before freeing the buffer object.

After switching to generic fbdev emulation and fixing the damage
handler, no perma-mapped buffers have to be released. Switch to SHMEM's
implementation of free_object.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191114141025.32198-3-tzimmermann@suse.de