drm/i915/dp: remove superfluous EXPORT_SYMBOL()

The symbol isn't needed outside of i915.ko.

Fixes: b30edfd8 ("drm/i915: Switch to LTTPR non-transparent mode link training")
Fixes: 264613b4 ("drm/i915: Disable LTTPR support when the DPCD rev < 1.4")
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210816071737.2917-1-jani.nikula@intel.com