nouveau: fix ambiguous backlight controls

If a display supports backlight control using the nouveau driver, and
also supports standard ACPI backlight control, there will be two sets of
controls.

/sys/class/backlight/acpi_video0
/sys/class/backlight/nv_backlight

This creates ambiguity because these controls can be out of sync with
each other.  One could be at 100% while the other is at 0% and the
actual display brightness depends on which one was used last.  This also
creates anomalies in Powertop which will show two values for brightness
with potentially different values.

Fix this ambiguity by having the nouveau driver only enable its
backlight controls if the standard ACPI controls are not present.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>