drm/msm/dsi-staging: add support for change in minimum voltage of regulator

Some regulators require change in their minimum voltage level
before the client removes its vote. This change checks if there is
a change in the minimum voltage level required and updates the
corresponding level before the regulator is disabled.

Change-Id: Ib62027a01c21ed12bab5cd221f39e089a3b054bb
Signed-off-by: Tharun Raj Soma <tsoma@codeaurora.org>