drm/i915/skl: Definition of SKL WM param structs for pipe/plane

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: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Pradeep Bhat <pradeep.bhat@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>