drm/i915/dp: limit sink rates based on rate

Get rid of redundant intel_dp_num_rates(). We can simply look at the
rate and limit based on that.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by:  <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171009092959.29021-3-jani.nikula@intel.com