iris: Add resolve on iris_flush_resource.

iris: Add resolve on iris_flush_resource.

The flush_resource hook is supposedly called when the resource content
needs to be made visible to external (okay, that's pretty vague). For
instance, it gets called before a surface gets handled to the window
system. So we need to resolve it if it's not resolved yet.

v2 (Ken):
 - Check mod_info in iris_flush_resource instead of ISL_AUX_USAGE_NONE
 - Drop my old broken resolve code from iris_resource_get_handle() now
   that Rafael's got it hooked up in the right place.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>