drm_fourcc: Document linear modifier

Not setting the fb modifiers flag is something different from setting
the fb modifiers to 0 (which means explicitly linear). We kinda failed
to document that properly. Spotted by Kristian.

Cc: hoegsberg@google.com
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1478694996-4200-1-git-send-email-daniel.vetter@ffwll.ch

include/uapi/drm/drm_fourcc.h

@@ -172,6 +172,16 @@ extern "C" {
  * authoritative source for all of these.
  */
 
+/*
+ * Linear Layout
+ *
+ * Just plain linear layout. Note that this is different from no specifying any
+ * modifier (e.g. not setting DRM_MODE_FB_MODIFIERS in the DRM_ADDFB2 ioctl),
+ * which tells the driver to also take driver-internal information into account
+ * and so might actually result in a tiled framebuffer.
+ */
+#define DRM_FORMAT_MOD_LINEAR	fourcc_mod_code(NONE, 0)
+
 /* Intel framebuffer modifiers */
 
 /*