intel: Attempt to fix up after "Update vertex texture code."

intel: Attempt to fix up after "Update vertex texture code."

The MaxCombinedTextureImageUnits is the total number of samplers that can
be bound between vertex, geometry, and fragment, not 0.  This should report
the correct value on 965 now.  Other DRI drivers may also need updating if
their MaxVertexTextureImageUnits != 0 (for example, if using the sw vertex
pipeline).

It's not clear to me if there's going to be a valid value for this
limit other than MaxTextureImageUnits + MaxVertexTextureImageUnits (+
MaxGeometryTextureImageUnits eventually).  If not, then we should probably
just move this into the core at Get time.

Bug #25518 (wine regression).  Fixes piglit vp-combined-image-units.