gxfb: fix incorrect __init annotation

WARNING: vmlinux.o(.data+0x195d8): Section mismatch in reference from the
variable gxfb_driver to the function .init.text:gxfb_probe() The variable
gxfb_driver references the function __init gxfb_probe()

This changes gxfb_probe and friends to use __devinit, and also adds
__devexit to gxfb_remove.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Cc: Jordan Crouse <jordan.crouse@amd.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>