drm: make drm_legacy.h self-contained

Do not require users of drm_legacy.h to include other
files just to let it build

Signed-off-by: Sam Ravnborg <[e-mail]>
Acked-by: Daniel Vetter <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: David Airlie <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_legacy.h

···
  * drivers use them, and removing them are API breaks.
  */
 #include <linux/list.h>
+
+#include <drm/drm.h>
+#include <drm/drm_device.h>
 #include <drm/drm_legacy.h>
 
 struct agp_memory;
 struct drm_device;
 struct drm_file;
+struct drm_buf_desc;
 
 /*
  * Generic DRM Contexts