drm/i915/selftests: Switch 4k kmalloc to use get_free_page for alignment

In generating the reference LRC, we want a page-aligned address for
simplicity in computing the offsets within. This then shares the
computation for the HW LRC which is mapped and so page aligned, making
the comparison straightforward. It seems that kmalloc(4k) is not always
returning from a 4k-aligned slab cache (which would give us a page aligned
address) so force alignment by explicitly allocating a page.

Reported-by: "Gote, Nitin R" <nitin.r.gote@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: "Gote, Nitin R" <nitin.r.gote@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200923114156.17749-1-chris@chris-wilson.co.uk