msm: mdss: Fix underrun while rotator is used

Underruns are observed on 8916 with rotator used, combined BW
of primary and rotator exceeds the threshold BW. Fail the
prepare call under such conditions so that fallback happens.

Change-Id: Ibe2110261ab6aeabe4e5e80532c0813c55d2c133
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>