drm/amd/display: Fix Werror when building

[Why & How]
Got Werror when building with Clang-13:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dpia.c:195:2:
error: variable 'ts' is used uninitialized whenever switch default is
taken [-Werror,-Wsometimes-uninitialized]

        default:

        ^~~~~~~

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dpia.c:200:9:
note: uninitialized use occurs here

        return ts;

               ^~

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dpia.c:180:2:
note: variable 'ts' is declared here

        enum dpia_set_config_ts ts;

        ^

1 error generated.

Fix it.

Reported-by: Mike Lothian <mike@fireburn.co.uk>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>