drm/radeon/dpm: fetch the max clk from voltage dep tables helper

This patch adds a helper function to fetch the max clock
from the voltage clock dependecy tables.  Clocks above that
level tend to be unstable and will require additional driver
tweaks in order to work properly.

This patch implemented the helper function to fetch the max clocks
from the dependency tables.  The following patches implement the
per-asic clock filtering.

See bug:
https://bugs.freedesktop.org/show_bug.cgi?id=68235

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