i965: Create a shader_dispatch_mode enum to replace VS/GS fields.

i965: Create a shader_dispatch_mode enum to replace VS/GS fields.

We used to store the GS dispatch mode in brw_gs_prog_data while
separately storing the VS dispatch mode in brw_vue_prog_data::simd8.

This patch introduces an enum to represent all possible dispatch modes,
and stores it in brw_vue_prog_data::dispatch_mode, unifying the two.

Based on a suggestion by Matt Turner.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>