drm/i915/dp: add helper for checking for UHBR link rate

Helpful abstraction to avoid duplication.

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/fe9a222ad900da797c989de9f7fa13928d2c9861.1631191763.git.jani.nikula@intel.com