fbdev: fix integer as NULL pointer warning

drivers/video/aty/atyfb_base.c:3359:26: warning: Using plain integer as NULL pointer
drivers/video/aty/radeon_base.c:2280:32: warning: Using plain integer as NULL pointer
drivers/video/matrox/matroxfb_base.h:203:25: warning: Using plain integer as NULL pointer
drivers/video/matrox/matroxfb_base.h:203:25: warning: Using plain integer as NULL pointer
drivers/video/sis/sis_main.c:5790:44: warning: Using plain integer as NULL pointer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>