i965: Advertise our vertex shader texture units.

i965: Advertise our vertex shader texture units.

Previously, we advertised 0 VS texture units.  Now that we have proper
support for using the sampling engine in the VS, we can advertise 16,
which is conveniently the number required for OpenGL 3.0.

v2: Enable on Gen4.  I hacked up my tests to not use flat ivec varyings
    and they pass.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>