drm/amd/pp: handle negative values when reading OD

Reading the sysfs files pp_sclk_od and pp_mclk_od return the
percentage difference between the VBIOS-provided default
frequency and the current (possibly user-set) frequency in
the highest SCLK and MCLK DPM states, respectively.

Writing to these files provides an easy mechanism for
setting a higher-than-default maximum frequency. We
normally only allow values >= 0 to be written here.

However, with the addition of pp_od_clk_voltage, we now
allow users to set custom DPM tables. If they then set
the maximum DPM state to something less than the default,
later reads of pp_*_od should return a negative value.
The highest DPM state is now less than the VBIOS-provided
default, so the percentage is negative.

The math to calculate this was originally performed with
unsigned values, meaning reads that should return negative
values returned meaningless data. This patch corrects that
issue and normalizes how all of the calculations are done
across the various hwmgr types.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Joseph Greathouse <Joseph.Greathouse@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>