This patch defines the structures needed for computation of
watermarks of pipes and planes for SKL.

v2: Incorporated Damien's review comments and removed unused fields
    in structs for future features like rotation, drrs and scaling.
    The skl_wm_values struct is now made more generic across planes
    and cursor planes for all pipes.

v3: implemented the plane/cursor split.

v4: Change the wm union back to a structure (Ville, Daniel)
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NPradeep Bhat <pradeep.bhat@intel.com>
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>