xorg-xserver: always enable forced outputs

If the user has gone to the effort of manually enabling an output in
the configuration file assume that they know what they're doing.

X.org Bug 14611 <http://bugs.freedesktop.org/show_bug.cgi?id=14611

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Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>