fbdev: make FB_BACKLIGHT a tristate

fbdev: make FB_BACKLIGHT a tristate

BACKLIGHT_CLASS_DEVICE is already tristate, but a dependency
FB_BACKLIGHT prevents it from being built as a module.  There
doesn't seem to be any particularly good reason for this, so
switch FB_BACKLIGHT over to tristate.

Signed-off-by: Rob Clark <[email protected]>
Tested-by: Arnd Bergmann <[email protected]>
Cc: Simon Horman <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Laurent Pinchart <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Ulf Magnusson <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Hans de Goede <[email protected]>
Signed-off-by: Bartlomiej Zolnierkiewicz <[email protected]>