msm: mdss: Add support to halt MDSS axi ports

Add a utility to halt MDSS axi ports. This utility can be called during
deep suspend, display off or for debugging purposes. On success it should
be assumed that AXI ports are in idle state and would not fetch any more
data.

Change-Id: Iaec2c7f56cf77ff3db0e634cb4ea843da023be9b
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>