drm: Create drm legacy driver header

And move a few legayc functions to start things over there.

It compiles ...

Inspired by a patch from Dave Airlie, but with a split between drm.ko
private legacy functions and stuff used by drivers.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/via/via_drv.h

@@ -25,6 +25,8 @@
 #define _VIA_DRV_H_
 
 #include <drm/drm_mm.h>
+#include <drm/drm_legacy.h>
+
 #define DRIVER_AUTHOR	"Various"
 
 #define DRIVER_NAME		"via"