disp: msm: sde: allow gdsc toggle when external vote is present

The presence of an external vote like secure vote
will lead to the device not going for a gdsc toggle
after crtc is disabled. Add support to detect the
presence of an external vote and allow gdsc toggle
after crtc is disabled for bengal target.

Change-Id: I23e29658a622245d8eb26c14606e1fabfe92d6af
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>