msm: vidc: Fix MBs per frame to derive slices per frame

Video driver calculates MBs per frame considering per MB
as 16x16. HEVC would have lcu size as 32 and hence per
MB would be 32x32. As a result of this, MBs per slice
is being set more.
Since there is a limit of number of MBs feasible to get
a slice within a tile, crossing the limit would eventually
end up with no slices.

Change-Id: I062655ac08b905c20afc913b99333365826fd2dd
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>