gallium: fix incorrect level param in copy_image_data_to_texture()

This fixes a weird conform test failure that depended on something that
happened several tests earlier.  This took waaaaaay too long to find/fix.

src/mesa/state_tracker/st_cb_texture.c

       st_texture_image_data(st->pipe,
                                stObj->pt,
                                stImage->face,
-                               stImage->level,
+                               dstLevel,
                                stImage->base.Data,
                                stImage->base.RowStride,
                                stImage->base.RowStride *