vidc: fix NV12 buffer size calculation in encoder and decoder.

Buffer size calculation in driver and firmware is different. This
results in inconsistent behaviour for different resolutions. This
change calculates buffer size as in firmware.

Change-Id: I9dca9e0e8729eb4e578de63dc43c01dd65c3f3be
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>