drm/amd/powerplay: set a default fclk/gfxclk ratio

Otherwise big gap between these two clocks may causes
some hangs.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>