meson: define empty variables for libswdri and libswkmsdri

meson: define empty variables for libswdri and libswkmsdri

This allows these variables to unconditionally included in `link_with`
lists, even if they're not used. This allows deleting duplicated logic
in nearly every gallium target implemented in meson today. This also
removes the now useless `build_by_default` flag from swdri and swkmsdri.

v4: - add this patch

Fixes: 66c94b9313a697ce8f2b222f4ba353035e4b8726
       ("meson: build gallium winsys for dri, null, and wrapper")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Acked-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>