drm: add cap bit to denote if dumb ioctl is available or not.

This allows libkms to make an easier decision.

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/include/drm/drm.h b/include/drm/drm.h
index da4efd1..9ac4313 100644
--- a/include/drm/drm.h
+++ b/include/drm/drm.h
@@ -752,6 +752,8 @@
 	__u32 reserved;
 };
 
+#define DRM_CAP_DUMB_BUFFER 0x1
+
 /* typedef area */
 #ifndef __KERNEL__
 typedef struct drm_clip_rect drm_clip_rect_t;