drm/amdgpu/powerplay: properly set PP_GFXOFF_MASK (v2)

So that the setting reflects what the hw supports. This will
be used in a subsequent patch so needs to be correct.

v2: squash in fix from Colin Ian King

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=205497


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