drm/i915/gem: Prevent using pgprot_writecombine() if PAT is not supported

Let's not try and use PAT attributes for I915_MAP_WC if the CPU doesn't
support PAT.

Fixes: 6056e500 ("drm/i915/gem: Support discontiguous lmem object maps")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Cc: <stable@vger.kernel.org> # v5.6+
Link: https://patchwork.freedesktop.org/patch/msgid/20200915091417.4086-2-chris@chris-wilson.co.uk
(cherry picked from commit 121ba69ffddc60df11da56f6d5b29bdb45c8eb80)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/gem/i915_gem_pages.c

@@ -254,6 +254,10 @@ static void *i915_gem_object_map(struct drm_i915_gem_object *obj,
 	if (!i915_gem_object_has_struct_page(obj) && type != I915_MAP_WC)
 		return NULL;
 
+	if (GEM_WARN_ON(type == I915_MAP_WC &&
+			!static_cpu_has(X86_FEATURE_PAT)))
+		return NULL;
+
 	/* A single page can always be kmapped */
 	if (n_pte == 1 && type == I915_MAP_WB) {
 		struct page *page = sg_page(sgt->sgl);