drm/msm/dsi-staging: fix bug handling dsi cmd done interrupt delays

Fix bug to get interrupt status value correctly by reading
the dsi interrupt status register. Otherwise if there are CPU
interrupt delays to handle the command done irq then will crash.

Change-Id: I5c2ea99105b7353cec01dec09636b60486d129dc
Signed-off-by: Vara Reddy <varar@codeaurora.org>