drm/i915/selftest: test aligned offsets for 64K

When using softpin it's not enough to just pad the vma size, we also
need to ensure the vma offset is at the start of the pt boundary, if we
plan to utilize 64K pages. Therefore to improve test coverage we should
use both aligned and unaligned gtt offsets in igt_write_huge.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181029203734.21936-1-matthew.auld@intel.com