The dri2 winsys also uses libdrm (and you can only enable dri3 if
you enable dri2), and the drm winsys only requires libdrm.

So if any winsys is enabled you can also enable the drm winsys, and
since we always want at least one winsys we can always enable it.

I removed the check for the drm platform for VA and OMX since they
do not care anymore. Since we still check for one of r600g, nouveau
or radeonsi, we are guarantueed to still only enable it by default
in a configuration that requires libdrm anyway. So for people using
va=auto, we don't suddenly start requiring libdrm were we did not
before.

This supersedes "vl: Enable DRM by default.", which I pushed, but
rolled back because it used dep_libdrm before its definition.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>