drm/msm/sde: control regulator only in normal mode

This change ensures that regualator is not controlled by
software driver once it is programmed as hw controlled.
Display RSC will put the mdss gdsc in hw controlled mode
and after that all sw clients should not enable/disable it.
Since it is guaranteed that sde driver will always put the
vote to turn on gdsc, this ensures that regulator is enabled
before any display hw access from clients.

Change-Id: I289f7c63a7b5a57b5a36fed31452ba0ab8808430
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>