msm: mdss: Fix hist sum sanity check issue

When dynamic resolution switch is enabled,
it may happen that mixer w/h get updated
for new resolution before hist for last frame
with different resolution is read. This can
lead to sanity check failure for hist sum.
Fix the issue by clearing the current expected
sum and caching sum for next frame.

Change-Id: I93734900770a6c97eac66a412710cae217835693
Signed-off-by: Rajesh Yadav <ryadav@codeaurora.org>