gallium: state tracker didn't always notify drivers of texobj data changes

gallium: state tracker didn't always notify drivers of texobj data changes

Calling glTexSubImage() or glTexImage() to replace texture data didn't
reliably cause pipe->set_sampler_texture() to get called so drivers didn't
always get notified of new texture data.
The st_texture_object->pt pointer doesn't always indicate changed data so
added a dirtyData field.