msm: mdss: Correct max threshold bandwidth selection logic

Max threshold Bandwidth selection logic should be independent of
number of entries in max bandwidth limit DT property. Correct the
logic to make it independent.

Change-Id: I6510ad7095560b25bbada7c63c44ae88a6d955f1
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>