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

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 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
b390ff35170fdc2b7f1fb1709a79d81edcd56981.

v2: (Caio)
 - Re-arrange code block.
 - Adjust comment.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6958

Fixes: f7262462 ("intel/fs: Rework fence handling in brw_fs_nir.cpp")
Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Tested-by: Mark Janes <markjanes@swizzler.org>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20996>
(cherry picked from commit 0c083d29a55bf20a993eab82786d583403582c8c)