i965/vs: Unify URB entry size/read length calculations between backends.

i965/vs: Unify URB entry size/read length calculations between backends.

Both the vec4 and scalar VS backends had virtually identical URB entry
size and read length calculations.  We can move those up a level to
backend-agnostic code and reuse it for both.

Unfortunately, the backends need to know nr_attributes to compute
first_non_payload_grf, so I had to store that in prog_data.  We could
use urb_read_length, but that's nr_attributes rounded up to a multiple
of two, so doing so would waste a register in some cases.

There's more code to be removed in the vec4 backend, but that will
come in a follow-on patch.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>