drm/amd/pm: ensure the fw_info is not null before using it

This resolves the dereference null return value warning
reported by Coverity.

Signed-off-by: Tim Huang <tim.huang@amd.com>
Reviewed-by: Jesse Zhang <jesse.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>