compositor: Mark pnode accordingly when buffer type is direct

As we exit early we never have the chance to mark it as is_direct when
the buffer type is direct_display.

This is required as is_direct is never checked in GL attach part we
end up tripping on invalid renderer_private causing it to assert.

Fixes 2db4f17244f, "compositor: re-order paint node placeholder checks"

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>