msm: mdss: Fix watermark level calculations for pipe updates

Watermark (wm) levels for active pipes are calculated and set for every
surface update regardless of pipe parameters are changed or not. In the
current implementation if reserved MMBs are not changed compared previous
frame then wm levels are reset to zero. This is not a correct behaviour
and may lead to under-runs. Fix this by setting wm levels based on
allocated MMBs.

Change-Id: I857c161f7203b64411e9345553f88c48f7726ee2
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>