drm/amd/display: Removed unused variable ret

Kernel test robot throws below warning ->

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link.c:
In function 'dc_link_reduce_mst_payload':
   drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link.c:3782:32:
warning: variable 'ret' set but not used [-Wunused-but-set-variable]
    3782 |         enum act_return_status ret;

Removed the unused ret variable.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Souptick Joarder (HPE) <jrdr.linux@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>