drm/i915/selftests: Wrap vm_mmap() around GEM objects

Provide a utility function to create a vma corresponding to an mmap() of
our device. And use it to exercise the equivalent of userspace
performing a GTT mmap of our objects.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107180601.30815-4-chris@chris-wilson.co.uk