drm/i915/gtt: flush the scratch page

The scratch page is directly visible in the users address space, and
while this is forced as CACHE_LLC, by the kernel, we still have to
contend with things like "Bypass-LLC" MOCS. So just flush no matter
what.

v2(Thomas):
  - Make sure we use drm_clflush_virt_range here, in case clflush support
    is missing.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211028092638.3142258-1-matthew.auld@intel.com

drivers/gpu/drm/i915/gt/intel_gtt.c

@@ -7,6 +7,8 @@
 
 #include <linux/fault-inject.h>
 
+#include <drm/drm_cache.h>
+
 #include "gem/i915_gem_lmem.h"
 #include "i915_trace.h"
 #include "intel_gt.h"
@@ -273,6 +275,7 @@ static void poison_scratch_page(struct drm_i915_gem_object *scratch)
 		val = POISON_FREE;
 
 	memset(vaddr, val, scratch->base.size);
+	drm_clflush_virt_range(vaddr, scratch->base.size);
 }
 
 int setup_scratch_page(struct i915_address_space *vm)