drm/radeon/dpm: use stored max_vddc rather than looking it up

When we parse the power tables use the stored mac_vddc value
rather than lookig it up manually each time.

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

diff --cc drivers/gpu/drm/radeon/ni_dpm.c
Simple merge

diff --cc drivers/gpu/drm/radeon/rv770_dpm.c
Simple merge