drm/amdgpu/pm: make gfxclock consistent for sienna cichlid

commit a4eb1182 upstream.

Use average gfxclock for consistency with other dGPUs.

Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 6.1.x
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>