decoder: h264: optimize support for grayscale surfaces.

decoder: h264: optimize support for grayscale surfaces.

Optimize support for grayscale surfaces in two aspects: (i) space
by only allocating the luminance component ; (ii) speed by avoiding
initialization of the (now inexistent) chrominance planes.

Keep backward compatibility with older codec layers that only
supported YUV 4:2:0 and not grayscale formats properly.

v2: fix check for extra H.264 chroma formats [Haihao]

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