change rivafb_remove to __devexit

Change rivafb_remove to __deviexit to fix MODPOST warnings:

WARNING: drivers/video/riva/rivafb.o - Section mismatch: reference to
.exit.text:rivafb_remove from .data.rel.local after 'rivafb_driver' (at offset 0x28)

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Acked-by: James Simmons <jsimmons@infradead.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>