vidc: send flush done if flush is issued in EoS while encoding

There is a race condition in encoder, when an input buffer
is received with an EoS flag but doesn't have an output
buffer to process that and if simultaneously flush is issued.
This is fixed by sending flush done if flush is issued in EoS.

CRs-Fixed: 353081
Change-Id: I925f49779ed5cb91bfab805ba4b7102c1e3f728b
Signed-off-by: Rajeshwar Kurapaty <rkurapat@codeaurora.org>