drm/i915/skl: Add DDB allocation management structures

We now need to allocate space in the DDB for planes being scanned out
ourselves. The data structure to represent an allocation mirrors what
we'll need to write in the registers later on: (start, end).

We add that allocation datat to the skl_wm_values structure as part of
the values to program the hardware with.

v2: Split planes and cursor for consistency.

v3: Make the skl_ddb_entry_size() parameter const

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