msm: vidc: Expose extradata size to userspace

Previously, the extradata size was included within VENUS_BUFFER_SIZE
and callers (primarily in userspace) wouldn't know how much extra
padding was added to the buffer size. Exposing it allows userspace to
query directly instead of doing guesswork.

Change-Id: I7f9701a4adfe364d757028514bdd4fa84402a995
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>