drm/amdgpu: Fix kernel NULL pointer dereference in dpm functions

caused by
'commit 83e3c4615872 ("drm/amdgpu: Remove wrapper layer of smu ip functions")'

BUG: unable to handle kernel NULL pointer dereference at 00000000000005d8
[  313.241459] IP: ci_dpm_read_sensor+0x37/0xf0 [amdgpu]

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>