drm: use NULL instead of 0

From: Randy Dunlap <rdunlap@xenotime.net>

Use NULL instead of 0 (sparse warnings):

drivers/char/drm/ati_pcigart.c:64:10: warning: Using plain integer as NULL
pointer
drivers/char/drm/ati_pcigart.c:130:21: warning: Using plain integer as NULL
pointer
drivers/char/drm/ati_pcigart.c:171:14: warning: Using plain integer as NULL
pointer

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>