msm: vidc: Increase buffer size by 4k

Firmware can allocate a few extra bytes (atmost 4k)
than the actual YUV buffer size. This change is to
make sure that the buffers allocated by userspace clients
always meet or exceed the video firmware requirements. If
buffer requirement is not met, video playback will fail.

CRs-fixed: 428758
Change-Id: I465a90ccc43cb983f5a11dfbd21ef9d86f1e4e83
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>