i965: Fix gl_MaxCombinedTextureImageUnits.

We've always overriden
ctx->Const.{Vertex,Fragment}Program.MaxTextureImageUnits to reflect
the number of texture image units supported by the hardware (rather
than using the default values assigned by Mesa core) so it seems
sensible to do that for GeometryProgram.MaxTextureImageUnits too.  We
set it to 0 if geometry shaders aren't supported.

Once that is done, we can just unconditionally add
GeometryProgram.MaxTextureImageUnits to MaxCombinedTextureImageUnits.

Fixes piglit test "spec/glsl-1.50/built-in
constants/gl_MaxCombinedTextureImageUnits".