drm: Put related statements next to each other in Makefile

Give the Makefile a bit more structure by putting rules for core,
helpers, drivers, etc next to each other.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220421073108.19226-2-tzimmermann@suse.de