drm/amd/display: Drop `ret` variable from dm_suspend()

The `ret` variable in dm_suspend() doesn't get set and is just used
to return 0.  Drop the needless declaration.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>