surface: fix geometry (size, layout) of grayscale surfaces.

surface: fix geometry (size, layout) of grayscale surfaces.

Fix size of the allocated buffer used to represent grayscale (Y800)
surfaces. Only the luminance component is needed, thus implying a
single plane.

Likewise, update render routines to only submit the first plane.
The existing render kernels readily only care about that single
plane.

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