Backlight class attributes are currently easy to implement incorrectly.
Moving certain handling into the backlight core prevents this whilst at the
same time makes the drivers simpler and consistent.  The following changes are
included:

The brightness attribute only sets and reads the brightness variable in the
backlight_properties structure.

The power attribute only sets and reads the power variable in the
backlight_properties structure.

Any framebuffer blanking events change a variable fb_blank in the
backlight_properties structure.

The backlight driver has only two functions to implement.  One function is
called when any of the above properties change (to update the backlight
brightness), the second is called to return the current backlight brightness
value.  A new attribute "actual_brightness" is added to return this brightness
as determined by the driver having combined all the above factors (and any
driver/device specific factors).

Additionally, the backlight core takes care of checking the maximum brightness
is not exceeded and of turning off the backlight before device removal.

The corgi backlight driver is updated to reflect these changes.
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>