FROMGIT: drm/amgpu/acp: Implement set_powergating_state for acp

so driver can powergate acp block after asic initialized
to save power.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

(cherry picked from commit c36628d8
git://anongit.freedesktop.org/drm/drm of origin/drm-next branch)

BUG=b:117711307
TEST=Build & Boot on Careena

Change-Id: I871c4822a72709afc4145323e83de318b1d71ed7
Reviewed-on: https://chromium-review.googlesource.com/1294555


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Hang Zhou <hang.zhou@amd.corp-partner.google.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>