drm/amdgpu: validate suspend before function call

Before making a function call to suspend, validate
the function pointer like we do in sw_init.

Use the helper function amdgpu_ip_block_suspend where
same checks and calls are repeated.

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>