i965/vs: Remove brw_vs_prog_data pointer from brw_vs_compile.

i965/vs: Remove brw_vs_prog_data pointer from brw_vs_compile.

In patches that follow, we'll be splitting structs brw_vs_prog_data
and brw_vs_compile into a vec4-generic base struct and a VS-specific
derived struct (this will allow the vec4-generic code to be re-used
for geometry shaders).  Having brw_vs_compile point to
brw_vs_prog_data makes it difficult to do this cleanly.

Fortunately most of the functions that use brw_vs_compile (those in
the vec4_visitor class) already have access to brw_vs_prog_data
through a separate pointer (vec4_visitor::prog_data).  So all we have
to do is use that pointer consistently, and plumb prog_data through
the few remaining functions that need access to it.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>