CVE-2026-68430 drm/amdgpu/gfx8: drop unecessary BUG_ON()

In the Linux kernel, the following vulnerability has been resolved: drm/amdgpu/gfx8: drop unecessary BUGON There's no need to crash the kernel for this case. cherry picked from commit 4d7c25208ca612b754f3bf39e9f16e725b828891...