msm: vidc: Add flush for video codecs

Add support to flush input/output buffers. This will flush
the buffers being held by firmware or video driver and
return them to client. This operation is usually used
by an application during seek operation or while
closing the driver.

Change-Id: I61d3d8f94eeeeeea0e4c40460bebfd7873f428df
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>