drm/radeon: use NULL instead of zero in clearstate headers

Signed-off-by: Michele Curti <michele.curti@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/clearstate_cayman.h
Simple merge

diff --cc drivers/gpu/drm/radeon/clearstate_ci.h
Simple merge

diff --cc drivers/gpu/drm/radeon/clearstate_si.h
Simple merge