msm: vidc: Remove vcd restriction of duplicate input buffer queue

VCD previously had a limitation of not allowing two consecutive
encode_frame calls with the same buffer. This was primarily due to the
handling of buffer_entry elements in the buffer pool.  This commit
removes this restriction by maintaining an internal buffer_entry element
that VCD keep track of.

Change-Id: If1788246444d055f9fc369ebd61f57a080d8d333
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>