drm/amdgpu: Remove GFXOFF MASK for Vangogh

 1. Remove PP_GFXOFF_MASK and then GFXOFF can be enabled
    by user space.
 2. GFXOFF is still disabled on Vangogh by default.
 3. When GFXOFF feature on Vangogh landed, will enable
    GFXOFF by default.
 4. GFXOFF can be enabled by debugfs interface amdgpu_gfxoff.

Signed-off-by: Jinzhou Su <Jinzhou.Su@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>