msm: vidc: Do not duplicate sequence header for secure sessions

In secure sessions, we aren't allowed write into secure buffers.  In
these cases, do not copy the sequence header from the internal buffer
into the output buffer. The firmware will prepend the sequence header to
the first I-frame in these cases.

Change-Id: I2e853210b6a566468ec152c6f80b411bc3e45848
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>