msm: mdss: handle DSI PHY regulator control for dual DSI cases

The DSI PHY regulator for both DSI0 and DSI1 is shared. So,
the regulator can be disabled during suspend only if both DSI0
and DSI1 are turned off, but both the DSI devices can be turned
on/off independently in case of dual DSI cases. Add change for
proper synchronization of DSI PHY regulator enable/disable.

Change-Id: I39b277d43e8b3bd1e7c475584da506566092c869
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>