drm/i915/dp: move link rate arrays where they're used

Localize link rate arrays by moving them to the functions where they're
used. Further clarify the distinction between source and sink
capabilities. Split pre and post Haswell arrays, and get rid of the
array size arithmetics. Use a direct rate value in the paranoia case of
no common rates find.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180227105911.4485-1-jani.nikula@intel.com