drm/radeon/kms: properly program vddci on evergreen+

Change vddci as well as vddc when changing power modes
on evergreen/ni.  Also, properly set vddci on boot up
for ni cards.  The vbios only sets the limited clocks
and voltages on boot until the mc ucode is loaded.  This
should fix stability problems on some btc cards.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>