drm/amdgpu: add gfx power gating for gfx10

This patch adds power gating handler for gfx10.

v2: simplify function

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