build: do not link libdrm without DRM backend

build: do not link libdrm without DRM backend

The pkg-config test for LIBDRM is independent of whether the DRM backend
is enabled or not. Therefore it is possible to have libdrm available and
found, even though it is not needed.

Do not link libdrm.so into the launchers unless it is really needed,
that is, DRM compositor is built. Otherwise you end up with
fbdev-backend.so and weston-launch depending on libdrm.so.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>