drm/i915: Nuke intel_mode_max_pixclk()

ilk_max_pixel_rate() will now give the "correct" pixel rate for all
platforms, so let's rename it to intel_max_pixel_rate() and kill
off intel_mode_max_pixclk().

v2: Fix typo in commit message (Ander)

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/20170120182205.8141-3-ville.syrjala@linux.intel.com