drm/amd/powerplay: no need to mask workable gfxoff feature for vega12

Gfxoff feature for vega12 is workable. So, there is no need to
mask it any more.

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