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.