msm: mdss: Add support for separate irq line for DSI 6G

In ferrum DSI has an irq line which is different from mdp.
Add support for irq handling this case. It makes use of
utility functions provided by mdss.

Change-Id: Ia6dca1050957f4755b7547f4c1a08ace913c2ac7
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>