drm/i915/skl: Allocate DDB portions for display planes

v2: Fix the 3rd plane/cursor logic (Pradeep Bhat)
v3: Fix one-by-one error in the DDB allocation code
v4: Rebase on top of the skl_pipe_pixel_rate() argument change
v5: Replace the available/start/end output parameters of
    skl_ddb_get_pipe_allocation_limits() by a single ddb entry constify
    a few arguments
    Make nth_active_pipe 0 indexed
    Use sizeof(variable) instead of sizeof(type)
    (Ville)
v6: Use the for_each_crtc() macro instead of list_for_each_entry()

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>