i915g: Add buffer write callback

src/gallium/drivers/i915simple/intel_winsys.h

@@ -150,6 +150,17 @@ struct intel_winsys {
    void (*buffer_unmap)(struct intel_winsys *iws,
                         struct intel_buffer *buffer);
 
+   /**
+    * Write to a buffer.
+    *
+    * Arguments follows pwrite(2)
+    */
+   int (*buffer_write)(struct intel_winsys *iws,
+                       struct intel_buffer *dst,
+                       const void *src,
+                       size_t size,
+                       size_t offset);
+
    void (*buffer_destroy)(struct intel_winsys *iws,
                           struct intel_buffer *buffer);
    /*@}*/