via_drm.h: move struct via_file_private definition to drivers/gpu/drm/via/via_drv.h

Fixes userspace compile error since list_head is not exported to userspace
headers.

Suggested by Emil Velikov <emil.l.velikov@gmail.com> at
https://lkml.org/lkml/2015/6/3/792

Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/via/via_drv.h

@@ -102,6 +102,10 @@ typedef struct drm_via_private {
 	uint32_t dma_diff;
 } drm_via_private_t;
 
+struct via_file_private {
+	struct list_head obj_list;
+};
+
 enum via_family {
   VIA_OTHER = 0,     /* Baseline */
   VIA_PRO_GROUP_A,   /* Another video engine and DMA commands */