msm: mdss: remove additional vert decimation when per pipe BW exceeds

Currently if per pipe BW exceeds pre-defined limit, we add additional
vertical decimation to reduce BW. But this has inverse effect on quality.
To mitigate this issue, decimate further only if user has already
requested decimation else return an appropriate error which user can
detect and act upon it.

CRs-Fixed: 637971
Change-Id: I1c3b913036b7a1680d6c99b03d42f1204f71f89f
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>