omxil: factorize FreeBuffers and AllocateBuffers

This also adds a missing call to decoder_DeletePicture when
deinitializing the component.

Signed-off-by: Martin Storsjö <martin@martin.st>
omxil: vout: Factorize fifo init/destroy

Signed-off-by: Martin Storsjö <martin@martin.st>
doc: remove outdated stub doc (fixes #11700)