msm: mdss: fix check for bwc and decimation

Decimation flag is not sufficient to prevent decimation from being
enabled. Instead check that decimation values are also not set.

Change-Id: Iab5fb4ef96649a2f28f4203643856b9ab9df4bee
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>