msm: vidc: Modify buffer size calculation for NV12

When NV12 color format is set, the buffer requirements are calculated
wrongly. Fix the buffer size calculation.

Change-Id: I5b32ce7e32ec6f21a7cef18f88c0199cb75b5b36
Signed-off-by: Rajeshwar Kurapaty <rkurapat@codeaurora.org>