video: msm: Defer histogram reset to workqueue

Currently, when an underrun occurs, a reset request is sent to the
histogram hardware. This has been observed to cause a hang in the histogram
collection cycle. The fix for this hang is to instead rely on the existing
checks for histogram validity in the histgram read worker. When the
underrun occurs, the histogram is invalidated, which in turn causes the
histogram read worker to reset the histogram collection when it attempts to
read its contents.

Change-Id: I66d33ddedef76fdcb6c4a3d8095c0e8ca695e33d
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>