i965: Add support for half-float formats.

i965: Add support for half-float formats.

Now that all the rest of the driver is driven off of the surface
formats table, all we really need to do is add the mapping from
MESA_FORMAT to BRW_SURFACEFORMAT.  However, we also add format
override for I16/L16 render targets at the same time, so that existing
users of I16 that were getting promoted to I32 and then getting the
I32->R32 override still get FBO support.

Fixes failures in piglit gl-3.0-required-sized-texture-formats, and
will prevent regressions in ARB_texture_float on gen4 when moving to
fully table-driven texture format setup.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>