drm/i915/dp: do not limit rate seek when not needed

In link training fallback, we're trying to find a rate that we know is
in a sorted array of common link rates. We don't need to limit the array
using the max rate. For test request, the DP CTS doesn't say we should
limit the rate based on earlier fallback. This lets us get rid of
intel_dp_link_rate_index() and use intel_dp_rate_index() instead.

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/33cab481a3228f31e938b5891a6285d892dcf272.1491485983.git.jani.nikula@intel.com