drm/i915: Drop expectations of VM_IO from our GGTT mmappings

We don't really want to use VM_IO for our GGTT mmaps (it implies that
the mmap contains memory mapped registers, which we do not expose) yet I
overzealously added it to an assert just because that's how we always
had setup the vma.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190807094128.9993-1-chris@chris-wilson.co.uk