drm/msm/dsi-staging: register dsi irq before splash handoff

In the current implementation dsi irq enable is tied to
dsi core clock enable and disable. But in continuous splash
scenario dsi core is getting enabled before dsi irq can be
registered. So when from core clock enable function, dsi irq
enable call is made, it is returning early since irq is not
registered yet and later this is resulting in irq storm if
any dsi irq is getting triggered. This change fixes the same
by registering dsi irq before splash handoff sequence begins.

Change-Id: I06528884667d63b25df5aa8276a34233315e8395
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>