drm: radeon: replace 0 with NULL

Replace 0 with NULL to fix sparse tool  warning
 warning: Using plain integer as NULL pointer

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>