xf86drm: implement drmParseSubsystemType for OpenBSD

Implement drmParseSubsystemType for OpenBSD by always returning
DRM_BUS_PCI.  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.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

xf86drm.c

@@ -2887,6 +2887,8 @@ 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;