staging: vc04_services: codec: Fix component enable/disable

start_streaming enabled the VPU component if ctx->component_enabled
was not set.
stop_streaming disabled the VPU component if both ports were
disabled. It didn't clear ctx->component_enabled.

If seeking, this meant that the component never got re-enabled,
and buffers never got processed afterwards.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>