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.

src/mesa/drivers/dri/i915/i915_context.c

@@ -143,6 +143,9 @@ i915CreateContext(const __GLcontextModes * mesaVis,
    ctx->Const.MaxTextureImageUnits = I915_TEX_UNITS;
    ctx->Const.MaxTextureCoordUnits = I915_TEX_UNITS;
    ctx->Const.MaxVarying = I915_TEX_UNITS;
+   ctx->Const.MaxCombinedTextureImageUnits =
+      ctx->Const.MaxVertexTextureImageUnits +
+      ctx->Const.MaxTextureImageUnits;
 
    /* Advertise the full hardware capabilities.  The new memory
     * manager should cope much better with overload situations:

src/mesa/drivers/dri/i965/brw_context.c

@@ -111,7 +111,9 @@ GLboolean brwCreateContext( const __GLcontextModes *mesaVis,
    ctx->Const.MaxTextureUnits = MIN2(ctx->Const.MaxTextureCoordUnits,
                                      ctx->Const.MaxTextureImageUnits);
    ctx->Const.MaxVertexTextureImageUnits = 0; /* no vertex shader textures */
-   ctx->Const.MaxCombinedTextureImageUnits = 0;
+   ctx->Const.MaxCombinedTextureImageUnits =
+      ctx->Const.MaxVertexTextureImageUnits +
+      ctx->Const.MaxTextureImageUnits;
 
    /* Mesa limits textures to 4kx4k; it would be nice to fix that someday
     */