drm/amdgpu/swsmu: add automatic parameter to set_soft_freq_range

On chips that support it, you can specificy 0 and 0xffff for
min and max and the PMFW will use that to determine the optimal
min and max.  This enables optimal performance when the
user manually switches between performance levels using sysfs.
Previously we'd set soft min/max which could limit performance.

Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>