drm/amd/display: Fix Null point error if smu ip was disabled

from AI, SMU Ip is not indispensable to driver and can be
disabled by user via module parameter ip_block_mask.
so the pp_handle may be NULL.

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>