msm: vidc: fix slice mbs setting

When MB_MODE slice is enabled, should use output
width/height to calculate the allowed slice size.
Fix warning log for NV21 format.

Change-Id: I97441cef36484ac6e82ba78b816f96d4b5bf2ab4
Signed-off-by: Qiwei Liu <qiweil@codeaurora.org>