drm/amdgpu: fix fence calculation (v2)

The proper metric for fence utilization over several
contexts is an harmonic mean, but such calculation is
prohibitive in kernel space, so the code approximates it.

Because the approximation diverges when one context has a
very small ratio compared with the other context, this change
filter out ratios smaller that 0.01%

v2: make the fence calculation static and initialize variables
within that function

v3: Fix warnings (Alex)

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: David M Nieto <david.nieto@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210513174539.27409-2-david.nieto@amd.com