ARM: tegra dvfs: Keep DVFS rate for disabled GPU clock

ARM: tegra dvfs: Keep DVFS rate for disabled GPU clock

Preserved DVFS rate for single clock GPU rail when clock is disabled
(instead of setting zero rate). In this case GPU rail is turned off
explicitly, anyway. However with non-zero DVFS rate voltage level at
regulator is appropriately updated when temperature is changes while
rail is off.

Bug 1526819

Change-Id: I022d908a47be81efbe37d8a777e93b1fec74e7e7
Signed-off-by: Alex Frid <afrid@nvidia.com>
Reviewed-on: http://git-master/r/426917
GVS: Gerrit_Virtual_Submit
Reviewed-by: Yu-Huan Hsu <yhsu@nvidia.com>