mdss: fix dynamic FPS mode switch configuration

For dual DSI panels, DFPS mode switch configuration through
sysfs is not properly enabled for the second DSI interface.
Fix this by initializing the mode switch update variable.

While changing the DFPS mode, idle-time should be disabled
and the device should be configured to default FPS. Add
these checks when DFPS mode switch is initiated.

Change-Id: Ib9e67f20758e2fb1c72a747c31109364db1d1160
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>