drm: Fix sparce warning in radeon driver

From: Luiz Fernando Capitulino <lcapitulino@mandriva.com.br>

drivers/char/drm/radeon_cp.c:1643:31: warning: Using plain integer as NULL
pointer

Signed-off-by: Luiz Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>