intel: make intel_flush_front safe to call during initial MakeCurrent

intel: make intel_flush_front safe to call during initial MakeCurrent

The patch that follows will fix a bug that prevents
intel_flush_front() from being called often enough.  In doing so, it
will create a situation where intel_flush_front() is called during the
initial call to glXMakeCurrent().  In this circumstance,
ctx->DrawBuffer hasn't been initialized yet and is NULL.  Fortunately,
intel->front_buffer_dirty is false, so intel_flush_front() doesn't
actually need to do anything.  To avoid a segfault, swap the order of
terms in intel_flush_front()'s if statement.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>