drm/amd/display: Fix -Wuninitialized in dm_helpers_dp_mst_send_payload_allocation()

When building with clang, there is a warning (or error when
CONFIG_WERROR is set):

  drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_helpers.c:368:21: error: variable 'old_payload' is uninitialized when used here [-Werror,-Wuninitialized]
    368 |                                                  new_payload, old_payload);
        |                                                               ^~~~~~~~~~~
  drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_helpers.c:344:61: note: initialize the variable 'old_payload' to silence this warning
    344 |         struct drm_dp_mst_atomic_payload *new_payload, *old_payload;
        |                                                                    ^
        |                                                                     = NULL
  1 error generated.

This variable is not required outside of this function so allocate
old_payload on the stack and pass it by reference to
dm_helpers_construct_old_payload(), resolving the warning.

Closes: https://github.com/ClangBuiltLinux/linux/issues/1931
Fixes: 5aa1dfcd ("drm/mst: Refactor the flow for payload allocation/removement")
Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230913-fix-wuninitialized-dm_helpers_dp_mst_send_payload_allocation-v1-1-2d1b0a3ef16c@kernel.org

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c

@@ -341,7 +341,7 @@ bool dm_helpers_dp_mst_send_payload_allocation(
 	struct amdgpu_dm_connector *aconnector;
 	struct drm_dp_mst_topology_state *mst_state;
 	struct drm_dp_mst_topology_mgr *mst_mgr;
-	struct drm_dp_mst_atomic_payload *new_payload, *old_payload;
+	struct drm_dp_mst_atomic_payload *new_payload, old_payload;
 	enum mst_progress_status set_flag = MST_ALLOCATE_NEW_PAYLOAD;
 	enum mst_progress_status clr_flag = MST_CLEAR_ALLOCATED_PAYLOAD;
 	int ret = 0;
@@ -365,8 +365,8 @@ bool dm_helpers_dp_mst_send_payload_allocation(
 		ret = drm_dp_add_payload_part2(mst_mgr, mst_state->base.state, new_payload);
 	} else {
 		dm_helpers_construct_old_payload(stream->link, mst_state->pbn_div,
-						 new_payload, old_payload);
-		drm_dp_remove_payload_part2(mst_mgr, mst_state, old_payload, new_payload);
+						 new_payload, &old_payload);
+		drm_dp_remove_payload_part2(mst_mgr, mst_state, &old_payload, new_payload);
 	}
 
 	if (ret) {