drm/i915/skl: Use a more descriptive parameter name in skl_compute_plane_wm()

What we're talking about here is the DDB allocation (in blocks). That's
more descriptive than 'max_page_buff_alloc'.

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>