drm/msm/dsi-staging: disable dsi irq before core clock off

Disable dsi irq before dsi core clock off, since dsi irq
disable sequence involves register access and this may lead
to NOC errors.

Change-Id: If0444c6da1c37ad0159080ff96e6b9bc892f98a9
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>