drm/mode: Add user blob-creation ioctl

Add an ioctl which allows users to create blob properties from supplied
data. Currently this only supports modes, creating a drm_display_mode from
the userspace drm_mode_modeinfo.

v2: Removed size/type checks.
    Rebased on new patches to allow error propagation from create_blob,
    as well as avoiding double-allocation.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@intel.com>
Tested-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drmP.h

@@ -326,6 +326,10 @@ struct drm_file {
 	struct list_head fbs;
 	struct mutex fbs_lock;
 
+	/** User-created blob properties; this retains a reference on the
+	 *  property. */
+	struct list_head blobs;
+
 	wait_queue_head_t event_wait;
 	struct list_head event_list;
 	int event_space;