drm: Build drm_memory.o only for legacy drivers

The file contains I/O-memory functions that are only used by legacy
drivers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112081035.6882-4-tzimmermann@suse.de