drm/amdgpu/powerplay: Only apply optimized mclk dpm policy on polaris

Leads to improper dpm on older parts.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1353


Fixes: 8d89b96f ("drm/amd/powerplay: optimize the mclk dpm policy settings")
Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>