drm/i915: Disable THP until we have a GPU read BW W/A

We seem to be missing some W/A for 2M pages and are getting
a hit on raw GPU read bandwidths (even 30%) even though the
GPU write bandwidths improve (even 10%).

For now, disable THP, which is our only practical source of
2M pages until we have a W/A for the issue.

v2:
- Be explicit that we talk about GPU bandwidths (Eero)
- s/deny/never/ because that's why (Chris)
Reported-by: Valtteri Rantala <valtteri.rantala@intel.com>
Fixes: b901bb89 ("drm/i915/gemfs: enable THP")
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Valtteri Rantala <valtteri.rantala@intel.com>
Cc: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Tested-by: Valtteri Rantala <valtteri.rantala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171127091233.7001-1-joonas.lahtinen@linux.intel.com

drivers/gpu/drm/i915/i915_gemfs.c

@@ -52,7 +52,8 @@ int i915_gemfs_init(struct drm_i915_private *i915)
 
 	if (has_transparent_hugepage()) {
 		struct super_block *sb = gemfs->mnt_sb;
-		char options[] = "huge=within_size";
+		/* FIXME: Disabled until we get W/A for read BW issue. */
+		char options[] = "huge=never";
 		int flags = 0;
 		int err;