drm/i915: Don't pass pipe_wm around so much

{skl,icl}_build_plane_wm() don't need to be passed the pipe_wm, so
don't. And skl_build_pipe_wm() can easily dig it out itself.

Cc: Neel Desai <neel.desai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190312205844.6339-9-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>