i965/draw: Use 3-channel formats for vertex fetch when possible.

i965/draw: Use 3-channel formats for vertex fetch when possible.

For a long time, several of the 3-channel vertex formats didn't exist so we
faked them with 4-channel versions.  Starting with Sandy Bridge, we can use
R16G16B16_FLOAT and 8 and 16-bit integer formats become available on
Haswell and Bay Trail.

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