drm/amd/display: init local variable to fix format errors

[WHY & HOW]
Uninitialized local variables will cause format checker complain
about them.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Yihan Zhu <Yihan.Zhu@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>