drm/i915/lmem: Bypass aperture when lmem is available

In the scenario where local memory is available, we have
rely on CPU access via lmem directly instead of aperture.

v2:
gmch is only relevant for much older hw, therefore we can drop the
has_aperture check since it should always be present on such platforms.
(Chris)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Chris P Wilson <chris.p.wilson@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: CQ Tang <cq.tang@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210427085417.120246-6-matthew.auld@intel.com