video: platinumfb: Add __devexit_p at necessary place

According to the comments in include/linux/init.h:

"Pointers to __devexit functions must use __devexit_p(function_name), the
wrapper will insert either the function_name or NULL, depending on the config
options."

We have __devexit annotation for platinumfb_remove(), thus add __devexit_p at
necessary place.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>