nouveau: Add in-kernel backlight control support

Several nvidia-based systems don't support backlight control via the
standard ACPI control mechanisms. Instead, it's necessary for the driver
to modify the backlight control registers directly. This patch adds
support for determining whether the registers appear to be in use, and
if so registers a kernel backlight device to control them. The backlight
can then be controlled via existing userspace tools.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>