decoder: h264: factor out allocation of reconstructed surfaces.

decoder: h264: factor out allocation of reconstructed surfaces.

Add new avc_ensure_surface_bo() helper function to factor out the
allocatiion and initialization processes of the reconstructed VA
surface buffer stores.

Keep preferred native format (NV12) and initialize chroma values
to 0.0 (0x80) when needed for "fake" grayscale (Y800) surfaces
implemented on top of existing NV12.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>