disp: msm: sde: avoid zero bus vote when display is active

In targets where rsc is not supported minimum vote is not
present which might result in command fetch failures when
there is null commit. Made change to have minimum vote in such
cases.

Change-Id: Id7b58611cc9af8999b7112093fcf235950c8a1ee
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>