drm/i915: Move ilk_pipe_pixel_rate() to intel_display.c

Move ilk_pipe_pixel_rate() next to its only caller
(intel_crtc_compute_pixel_rate()).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170120182205.8141-15-ville.syrjala@linux.intel.com