drm/i915: Simplify intel_sanitize_enable_ppgtt

Remove dead code around has_aliasing_ppgtt condition.

Suggested-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010143355.16577-1-joonas.lahtinen@linux.intel.com