dri: Don't build libdricommon.la if we don't need it

dri: Don't build libdricommon.la if we don't need it

Refine 80aa78142d12b21dd7d4f0edc786af98a159a80f "dri: make sure to build libdricommon.la"
so we don't build libdricommon if we aren't building a dri driver which needs it (i.e.
if we are just building swrast)

In particular, this restores the ability to build the swrast dri driver without having to
have a xf86drm.h

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>