msm: mdss: Add support to halt pipe fetches

Add a utility to halt any pipe fetches and wait on it until all fetches
are completely halted. Use this utility during staging and un-staging
of the pipe.

Change-Id: Id865970063dffc2870be5fd2cf51b9711134dc89
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>