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

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: 5aa1dfcdf0a4 ("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