drm/selftests/test-drm_dp_mst_helper: Move 'sideband_msg_req_encode_decode' onto the heap

There is too much data being stored on the stack.

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c: In function ‘sideband_msg_req_encode_decode’:
 drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c:168:1: warning: the frame size of 1072 bytes is larger than 1024 bytes [-Wframe-larger-than=]

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-31-lee.jones@linaro.org

drivers/gpu/drm/selftests/test-drm_dp_mst_helper.c

@@ -122,7 +122,7 @@ sideband_msg_req_encode_decode(struct drm_dp_sideband_msg_req_body *in)
 {
 	struct drm_dp_sideband_msg_req_body *out;
 	struct drm_printer p = drm_err_printer(PREFIX_STR);
-	struct drm_dp_sideband_msg_tx txmsg;
+	struct drm_dp_sideband_msg_tx *txmsg;
 	int i, ret;
 	bool result = true;
 
@@ -130,8 +130,12 @@ sideband_msg_req_encode_decode(struct drm_dp_sideband_msg_req_body *in)
 	if (!out)
 		return false;
 
-	drm_dp_encode_sideband_req(in, &txmsg);
-	ret = drm_dp_decode_sideband_req(&txmsg, out);
+	txmsg = kzalloc(sizeof(*txmsg), GFP_KERNEL);
+	if (!txmsg)
+		return false;
+
+	drm_dp_encode_sideband_req(in, txmsg);
+	ret = drm_dp_decode_sideband_req(txmsg, out);
 	if (ret < 0) {
 		drm_printf(&p, "Failed to decode sideband request: %d\n",
 			   ret);
@@ -166,6 +170,7 @@ sideband_msg_req_encode_decode(struct drm_dp_sideband_msg_req_body *in)
 
 out:
 	kfree(out);
+	kfree(txmsg);
 	return result;
 }