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