drm/msm/dsi-staging: register dsi interrupt handler before mode switch

There can be a use case where during display resume, dynamic mode
switch flag might be set. In this scenario before registering for
interrupt handler, DSI driver will enable the clocks, which might
lead to interrupt storm as DSI won't be able to handle the interrupts.
This change fixes the same by moving interrupt handler registration
sequence before mode switch call.

Change-Id: I1a112175acc0c6883ca4d6ca4e535de7361499ff
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>