drm/amd/display: remove redundant null pointer check before kfree

kfree has taken null pointer into account. hence it is safe to remove
the unnecessary check.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/hdcp/hdcp_msg.c

@@ -174,8 +174,6 @@ static bool hdmi_14_process_transaction(
 			link->ctx,
 			link,
 			&i2c_command);
-
-	if (buff)
 	kfree(buff);
 
 	return result;