drm: make drm/drm_legacy.h self-contained

Do not require users of include/drm/drm_legacy.h to
include other files just to let it build.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20190526173535.32701-3-sam@ravnborg.org

include/drm/drm_legacy.h

 #ifndef __DRM_DRM_LEGACY_H__
 #define __DRM_DRM_LEGACY_H__
-
-#include <drm/drm_auth.h>
-#include <drm/drm_hashtab.h>
-
-struct drm_device;
-
 /*
  * Legacy driver interfaces for the Direct Rendering Manager
  *
@@ -39,6 +33,12 @@ struct drm_device;
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#include <drm/drm.h>
+#include <drm/drm_auth.h>
+#include <drm/drm_hashtab.h>
+
+struct drm_device;
+struct file;
 
 /*
  * Legacy Support for palateontologic DRM drivers