backlight: generic_bl: Remove unused function

genericbl_limit_intensity() is exported, but it is never
called anywhere else.

Fix the following sparse warning:

drivers/video/backlight/generic_bl.c:59:6:
  warning: symbol 'genericbl_limit_intensity' was not declared.
  Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>