msm: mdss: distinguish between mdp and dsi bus votes

For DSI read or write commands, they will also go through
axi bus.  Earlier, we were only voting for the mdp bus bandwidth.
In the case where the mdp bus bandwidth vote is 0 and there is
a DSI transaction, it will result in DSI hang.

Change-Id: Iafd9e06ae2050665392c6c74fece8f20cb6d44b1
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>