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

drivers/gpu/drm/i915/display/intel_dp_link_training.c

@@ -206,7 +206,6 @@ int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
 
 	return lttpr_count;
 }
-EXPORT_SYMBOL(intel_dp_init_lttpr_and_dprx_caps);
 
 static u8 dp_voltage_max(u8 preemph)
 {