drm/amdgpu: Correct the read sclk for navi10

According to hw, after navi10,it runs in dfll mode, and should
read sclk from AverageGfxclkFrequency.

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