hal: separate transcode loopback usecase for Tx and Rx

- In transcode loopback usecase source device is fixed
  with HDMI device and backend is not configurable.
- Separate transcode loopback usecase for Rx and Tx
  to select the backend based on the source device.

Change-Id: Icbed33fe748ab5f9fc4ace3ada605ba25ad2c51c
Signed-off-by: Surendar Karka <skarka@codeaurora.org>