drm/amdgpu: move missed gfxoff entry into amdgpu_gfx header

Move missed gfxoff entry to amdgpu_gfx.h.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>