drm/i915/gen8: Flip the 48b switch

Use 48b addresses if hw supports it (i915.enable_ppgtt=3).
Update the sanitize_enable_ppgtt for 48 bit PPGTT mode.

Note, aliasing PPGTT remains 32b only.

v2: s/full_64b/full_48b/. (Akash)
v3: Add sanitize_enable_ppgtt changes until here. (Akash)
v4: Update param description (Chris)

Cc: Akash Goel <akash.goel@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_params.c b/drivers/gpu/drm/i915/i915_params.c
index ca9b8f6..368df67 100644
--- a/drivers/gpu/drm/i915/i915_params.c
+++ b/drivers/gpu/drm/i915/i915_params.c
@@ -111,7 +111,7 @@
 module_param_named_unsafe(enable_ppgtt, i915.enable_ppgtt, int, 0400);
 MODULE_PARM_DESC(enable_ppgtt,
 	"Override PPGTT usage. "
-	"(-1=auto [default], 0=disabled, 1=aliasing, 2=full)");
+	"(-1=auto [default], 0=disabled, 1=aliasing, 2=full, 3=full with extended address space)");
 
 module_param_named_unsafe(enable_execlists, i915.enable_execlists, int, 0400);
 MODULE_PARM_DESC(enable_execlists,