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

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