msm: mdss: read active line count before triggering BTA

In the current implementation before triggering BTA,
DSI SW waits for extra time to skip display blanking period.
Add an additional check for active line count to ensure that
BTA is always triggered during display active region only.

Change-Id: Ife67f5a38fa9e8df6f8431e9d2b0179c207adeb2
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>