media: verisilicon: Free post processor buffers on error

When initializing the post processor, it allocates the same number of
buffers as the buf queue.
As the init function is called in streamon(), if an allocation fails,
streamon will return an error and streamoff() will not be called, keeping
all post processor buffers allocated.

To avoid that, all post proc buffers are freed in case of an allocation
error.

Fixes: 26711491 ("media: verisilicon: Refactor postprocessor to store more buffers")
Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>