drm: radeon: fix sparse integer as NULL pointer warnings in radeon_mem.c

drivers/char/drm/radeon_mem.c:91:23: warning: Using plain integer as NULL pointer
drivers/char/drm/radeon_mem.c:116:28: warning: Using plain integer as NULL pointer
drivers/char/drm/radeon_mem.c:124:28: warning: Using plain integer as NULL pointer
drivers/char/drm/radeon_mem.c:177:26: warning: Using plain integer as NULL pointer
drivers/char/drm/radeon_mem.c:177:53: warning: Using plain integer as NULL pointer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>