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>