drm/amd/display: fix use of uninitialized variable

tg_inst may be used uninitialized, so initialize it to 0.

Signed-off-by: Martin Leung <martin.leung@amd.com>
Reviewed-by: Jaehyun Chung <Jaehyun.Chung@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>