msm: vidc: Add input buffer flushing support

Flushing the core is now done with the V4L2_DEC_QCOM_CMD_FLUSH ioctl
rather than V4L2_DEC_CMD_STOP.  The latter is confusing and the
arguments specified by the V4L2 spec do not allow for flushing the input
and and output buffers seperately.

Change-Id: I53e18e06c4e09ee8496bdcc337b4798f22911708
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>