msm: mdss: enable DSI interrupts properly during device boot

In current implementation if continuous splash screen is
enabled, then for 8909 case DSI interrupts are not getting
enabled during boot up. This code change makes an explicit
call to DSI event to ensure DSI is initialised properly
during boot up.

Change-Id: I12a7e64b2945432f9b180b30f192f201684a2c65
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>