i965/vs: Use a fixed set of register classes.

i965/vs: Use a fixed set of register classes.

Arrays, structures, and matrices use large VGRFs of arbitrary sizes.
However, split_virtual_grfs() breaks those down into VGRFs of size 1.

For reference, commit 5d90b988791e51cfb6413109271ad102fd7a304c is the
analogous change to the FS backend.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>