drm/msm/sde: fix mdp ib bw vote calculations

Current driver divides the ib bandwidth vote based on
the number of dram channels; this is a wrong assumption
as the division needs to be only based in the number of
paths used for voting. This causes that the llcc as well
as the ebi votes are wrongly divided by two.
This change fixes this issue by dividing the ib vote
into the number of voting paths as well as removes
extra 400MBps that were added as nrt path.

Change-Id: Ibc18300aa4204478ebce8b64c88071bc014790ab
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>