drm/amd/display: remove unused variable 'ret' in dm_suspend()

Fix the following coccicheck warning:

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:1574:5-8: Unneeded
variable: "ret". Return "0" on line 1586

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>