msm_vidc: venc: set slice size to firmware.

Slice size information was not propogated to firmware resulting
in slice size = 0 for all slice enabled encodings. This change
configures proper size to firmware.

Change-Id: Ie87ac63791e3473aab98a7186364befbb2156128
Signed-off-by: Ashray Kulkarni <ashrayk@codeaurora.org>