drm/amdgpu: One function call less in amdgpu_cgs_acpi_eval_object() after error detection

The kfree() function was called in one case by the
amdgpu_cgs_acpi_eval_object() function during error handling
even if the passed variable "obj" contained a null pointer.

* Adjust jump targets according to the Linux coding style convention.

* Delete unnecessary initialisations for the variables "obj"
  and "params" then.

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>