drm/amdgpu: explicitly set pg_flags for ST

No need to retain previous settings as this is the first time
we set pg_flags.  Probably a copy/paste typo from the CZ code.
Avoids confusion.

No change in behavior as adev is kzallocated.

Reviewed-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>