drm/amdgpu: Clean the functions pointer set as NULL

We dont need to set the functions to NULL which arent
needed as global structure members are by default
set to zero or NULL for pointers.

Cc: Leo Liu <leo.liu@amd.com>
Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>