drm/amdgpu: Change assignment for a variable in amdgpu_cgs_acpi_eval_object()

Indicate successful function execution only at the end.
Thus omit initialisation for the variable "result" at the beginning.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>