drm/amdgpu: enable gfxoff code path for navi14

Based on navi10 gfxoff logic, enable the related code
path for navi14.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>