drm/i915: Constify some watermark data

hsw_pipe_wm_parameters and hsw_wm_maximums typically are read only. Make
them const.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>