msm: mdss: Fix out of bound read access in pipe_lookup

Have a local copy of the max_rect value from the pipe so as to avoid
out of bound read access as the pipe pointer is looped in the lookup
function.

Change-Id: Icc9552eb5a4b467dc83b03c08271752618ed79c8
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>