drm/amd/powerplay: Use bitwise instead of arithmetic operator for flags

This silences the following coccinelle warning:

"WARNING: sum of probable bitmasks, consider |"

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>