input: synaptics_dsx: fix compilation warning when !CONFIG_PM_*

When CONFIG_PM is not selected in defconfig, following warnings
are reported during driver compilation:

CC drivers/input/touchscreen/synaptics_dsx/synaptics_dsx_core.o
drivers/input/touchscreen/synaptics_dsx/synaptics_dsx_core.c:109:12:
warning: 'fb_notifier_callback' used but never defined
[enabled by default]
error, forbidden warning: synaptics_dsx_core.c:109
make[6]: *** [drivers/input/touchscreen/synaptics_dsx/synaptics_dsx_core.o]
Error 1

Since frame-buffer notifiers do not have any dependency on CONFIG_PM's
selection in defconfig, modify the driver accordingly to fix these
warnings.

Change-Id: Ie5f68a725639d40b6c3aabe965e55f64c79590ca
Signed-off-by: Himanshu Aggarwal <haggarwa@codeaurora.org>