drm/amdgpu: remove cgs_acpi_method_argument member method_length

It was redundant with data_length, and in fact set incorrectly in one case
leading to an out-of-bound read by memcpy in acpi_ut_copy_esimple_to_isimple,
reported by CONFIG_KASAN=y.

Signed-off-by: Nicolai Hähnle <Nicolai.Haehnle@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>