drm/i915: Add tiled framebuffer modifiers

To be used from the new addfb2 extension.

v2:
- Drop Intel-specific untiled modfier.
- Move to drm_fourcc.h.
- Document layouts a bit and denote them as platform-specific and not
  useable for cross-driver sharing.
- Add Y-tiling for completeness.
- Drop special docstring markers to avoid confusing kerneldoc.

v3: Give Y-tiling a unique idea, noticed by Tvrtko.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v1)
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 3da3dc5..99b2592 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -31,6 +31,7 @@
 #define _I915_DRV_H_
 
 #include <uapi/drm/i915_drm.h>
+#include <uapi/drm/drm_fourcc.h>
 
 #include "i915_reg.h"
 #include "intel_bios.h"