drm/msm/sde: add function to return raw interrupt status

Current hardware interrupt driver does not support reading of
raw interrupt status using interrupt index.  This patch adds
new function to perform atomic read and clear of interrupt status
using interrupt index returned from interrupt lookup function.

Change-Id: I0cb7f5802df6da270738e98bf4a1748978cdd565
Signed-off-by: Alan Kwong <akwong@codeaurora.org>