drm/i915/selftests: Fill all the drm_vma_manager holes

To test mmap_offset_exhaustion, we first have to fill the entire vma
manager leaving a single page. Don't assume that the vma manager is not
already fragment, and fill all the holes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111122706.28292-2-chris@chris-wilson.co.uk