msm: mdss: Prevent multiple completions for each read request

Each read request should only capture one histogram done. completion to
prevent future histograms from trying to read unavailable data. Makes sure
that the read request is set for both pipes before waiting for a new
histogram. Likewise, make sure the read request is unset for both pipes
after waiting. To prevent accumulation of histogram bins, only kick off
histograms when histogram state is IDLE (i.e. read or reset(recently
 zeroed) bins).

Change-Id: Idc1ebb14dce500a6e5dfc0261ecec992e727a380
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>