drm/amdgpu/pm: Fix the null pointer dereference for smu7

optimize the code to avoid pass a null pointer (hwmgr->backend)
to function smu7_update_edc_leakage_table.

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>