msm: mdss: trigger BTA during active region of display

If DSI SW requests BTA very close to end of blanking or
start of active region and the BTA acknowledgment takes
more then one BLLP time to complete, then it will have side
effects on video timing generator since DSI will not be
receiving pixel data until BTA finishes. So always wait for
video done interrupt and skip blanking to ensure we fall into
active region before sending BTA request. This will
ensure BTA request is always sent during active region.

Change-Id: Ia9e1193f0229df8c552c3ba0287a5a60837ae540
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>