msm: mdss: handle WB2 irq for MDSS 1.8.0

In MDSS 1.8.0, bit 2 in intr enable and intr status register
is marked for WB2 which is different from the other targets.
Add changes to handle the same.

Change-Id: I76947bc218c897e121bb22fbda7aaa57723ec708
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>