If CONFIG_BACKLIGHT_CORGI is not selected, then corgibl_limit_intensity()
is not present.  However, both corgi_pm.c and sharp_pm.c reference this
symbol, resulting in a link error.

Wrap the references with the relevant ifdefs, and avoid the resulting
NULL pointer dereference by making the code in sharpsl_pm.c also
conditional on the config symbol.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>