drm/amdgpu: Replace rcu_assign_pointer() with RCU_INIT_POINTER()

The rcu_assign_pointer() ensures that the initialization of a structure
is carried out before storing a pointer to that structre. It is always
safe to use RCU_INIT_POINTER() to NULL a pointer, instead of
rcu_assign_pointer().
This results in slightly smaller/faster code.

The following semantic patch was used:
<smpl>

@@
@@

- rcu_assign_pointer
+ RCU_INIT_POINTER
  (..., NULL)

</smpl>

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Muhammad Falak R Wani <falakreyaz@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>