drm/radeon/dpm: fix vddci setup typo on cayman

We were using the vddc mask rather than the vddci mask.

Bug:
https://bugzilla.kernel.org/show_bug.cgi?id=79071

May also fix:
https://bugs.freedesktop.org/show_bug.cgi?id=69723



Noticed by: Dieter Nützel <Dieter@nuetzel-hh.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org