msm: mdss: Correct DSC panel dimension when dest-scaler enabled

When destination scaler is enabled, DSC panel dimension should base on
the scaler destination dimension, and not using the mixer output
dimension. Also move DSC dimension validation after destination scaler
setting applied successfully.

CRs-Fixed: 1065274
Change-Id: Iee328c847ffc16154e78682515454be6a61b35b4
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>