i965: Change type of brw_context.primitive from GLenum to hardware primitive

i965: Change type of brw_context.primitive from GLenum to hardware primitive

For example, GL_TRIANLGES is converted to _3DPRIM_TRILIST.

The conversion is necessary because HiZ and MSAA resolve operations emit
a 3DPRIM_RECTLIST, which cannot be conveyed by GLenum.

As a consequence, brw_gs_prog_key.primitive is also converted.

v2
----
- [anholt] Split brw_set_prim into brw/gen6 variants in previous commit,
  since not much code is really shared between the two.
- [anholt] Replace switch statements with table lookups, since this is
  a hot path.

Reviewed-by: Eric Anholt <eric@anho.net>
Signed-off-by: Chad Versace <chad@chad-versace.us>