Implement drmParseSubsystemType for OpenBSD.

Always return DRM_BUS_PCI for now.  No non-pci drm drivers are in the
kernel and this is unlikely to change anytime soon as the existing ones
aren't permissively licensed.

ok kettenis@

lib/libdrm/xf86drm.c

@@ -2905,6 +2905,8 @@ static int drmParseSubsystemType(int maj, int min)
         return DRM_BUS_PCI;
 
     return -EINVAL;
+#elif defined(__OpenBSD__)
+	return DRM_BUS_PCI;
 #else
 #warning "Missing implementation of drmParseSubsystemType"
     return -EINVAL;