msm: mdss: Access DSI registers before turning off clock

BTA complete interrupt comes and turns off the clocks. In another
thread we read the panel status and DSI ack returns error. In the
error condition we are trying to read some DSI registers even
when the clocks are turned off.

Change-Id: Ica77c16f3cf4b69f46c9003bb299c6413e199468
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>