drm/i915/dp: use sizeof for memset instead of magic value

intel_dp_reset_link_train(struct intel_dp *intel_dp, uint32_t *DP,

			uint8_t dp_train_pat)

{

	memset(intel_dp->train_set, 0, 4);

	memset(intel_dp->train_set, 0, sizeof(intel_dp->train_set));

	intel_dp_set_signal_levels(intel_dp, DP);

	return intel_dp_set_link_train(intel_dp, DP, dp_train_pat);

}