drm/i915: don't conflate is_dgfx with fake lmem

When using fake lmem for tests, we are overriding the setting in
device info for dgfx devices. Current users of IS_DGFX() except one are
correct. However, as we add support for DG1, we are going to use it in
additional places to trigger dgfx-only code path.

In future if we need we can use HAS_LMEM() instead of IS_DGFX() in the
places that make sense to also contemplate fake lmem use.

v2: update gen8_gmch_probe() to use HAS_LMEM(): we need to steal the
mappable aperture later(which is fine since it doesn't exist on "DGFX"),
and use it as a substitute for LMEMBAR. The !mappable aperture property
is also useful since it exercises some other parts of the code too.
(Matthew Auld)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201001063917.3133475-1-lucas.demarchi@intel.com