drm/radeon: Fix warning using plain integer as NULL

sparse static analysis tools generate a warning with this message
"Using plain integer as NULL pointer". In this case this warning is
being shown because we are trying to intialize a pointer to NULL using
integer value 0.

Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>