drm/amdgpu: Delete an unnecessary variable initialisation in amdgpu_cgs_acpi_eval_object()

The variable "argument" will be set to an appropriate value a bit later.
Thus omit the explicit initialisation 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>