drm/i915/dp: rename rate_to_index() to intel_dp_rate_index() and reuse

drm/i915/dp: rename rate_to_index() to intel_dp_rate_index() and reuse

Rename the function, move it at the top, and reuse in
intel_dp_link_rate_index(). If there was a reason in the past to use
reverse search order here, there isn't now.

The names may be slightly confusing now, but intel_dp_link_rate_index()
will go away in follow-up patches.

v2: Use name intel_dp_rate_index (Dhinakaran)

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/c7b6197aaa12e368a0d024dc142fa574fd0443a7.1490712890.git.jani.nikula@intel.com