intel: Fix crash in automatic mipmap generation for glCopyTex{Sub,}Image.

The images aren't mapped at this point, so we want the generic Mesa path for
GenerateMipmapEXT that does the mapping/unmapping for us.  Ideally Mesa would
just call it for us.

src/mesa/drivers/dri/intel/intel_tex_copy.c

@@ -167,7 +167,7 @@ do_copy_texsubimage(struct intel_context *intel,
 
    /* GL_SGIS_generate_mipmap */
    if (intelImage->level == texObj->BaseLevel && texObj->GenerateMipmap) {
-      intel_generate_mipmap(ctx, target, texObj);
+      ctx->Driver.GenerateMipmap(ctx, target, texObj);
    }
 
    return GL_TRUE;