drm/i915: Rename hsw_wm_get_pixel_rate to ilk_pipe_pixel_rate

hsw_wm_get_pixel_rate() isn't specific to HSW. In fact it should be made
to handle all gens, but for now it depends on the PCH panel fitter
state, so give it an ilk_ prefix.

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>