drm/amdgpu/gfx9: update pg_flags after determining if gfx off is possible

We need to set certain power gating flags after we determine
if the firmware version is sufficient to support gfxoff.
Previously we set the pg flags in early init, but we later
we might have disabled gfxoff if the firmware versions didn't
support it.  Move adding the additional pg flags after we
determine whether or not to support gfxoff.

Fixes: 00544006

 ("drm/amdgpu: enable gfxoff again on raven series (v2)")
Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Tested-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Kai-Heng Feng <kai.heng.feng@canonical.com>
Cc: stable@vger.kernel.org