i965: Clean up context constant initialization code.

i965: Clean up context constant initialization code.

This was getting pretty out of hand, and with compute partially in place
and tessellation on the way, it was only going to get worse.

This patch makes a "stage exists?" predicate and a "number of stages"
count and uses them to clean up a lot of calculations.  We can just
loop over shader stages and set things for the ones that exist.  For
combined counts, we can just multiply by the number of stages.

It also tries to organize a little bit.

We should probably use _mesa_has_geometry_shaders/tessellation/compute
here, but we can't because ctx->Version isn't initialized yet.  Perhaps
that could be fixed in the future.

No change in "glxinfo -l" on Broadwell.

v2: Drop stray compute shader hunk.  Mark stage_exists as const.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>