drm/amdgpu: prevent NULL pointer dereference if ATIF is not supported

In the Linux kernel, the following vulnerability has been resolved: drm/amdgpu: prevent NULL pointer dereference if ATIF is not supported acpi_evaluate_object() may return AE_NOT_FOUND (failure), which would result in dereferencing buffer.pointer (obj) while being NULL. Although this case may be unrealistic for the current code, it is still better to protect against possible bugs. Bail out also when status is AE_NOT_FOUND. This fixes 1 FORWARD_NULL issue reported by Coverity Report: CID 1600951: Null pointer dereferences (FORWARD_NULL) (cherry picked from commit 91c9e221fe2553edf2db71627d8453f083de87a1)