BugLink: http://bugs.launchpad.net/bugs/1546572

VCE DPM works similarly to SCLK DPM.  Add a similar interface
for VCE for forcing the VCE clocks.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 044c0629)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>