intel/fs: Always stall between the fences on Gen11+

Be conservative in Gfx11+ and always stall in a fence. Since there are two different fences and the shader might want to synchronize between them.

This change also brings back the original code block for the stall between the fence and comment from the commit b390ff35.

Closes: #6958 (closed)

Fixes: f7262462 ("intel/fs: Rework fence handling in brw_fs_nir.cpp")

Thanks, @cmarcelo, and @majanes for helping me to understand/test things out.