[AMDGPU] Temporarily disable FeatureBackOffBarrier for GFX11

Enabling this feature exposed some incorrect codegen, where a workgroup-
scope barrier fails to properly synchronise two waves from the same
workgroup running on different SIMDs of the same CU.

Disabling FeatureBackOffBarrier causes an s_waitcnt to be emitted before
the barrier which works around the problem.

Differential Revision: https://reviews.llvm.org/D141379