i965/vs: split brw_vs_prog_data into generic and VS-specific parts.

i965/vs: split brw_vs_prog_data into generic and VS-specific parts.

This will allow the generic parts to be re-used for geometry shaders.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
v2: Put urb_read_length and urb_entry_size in the generic struct.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>