drm/i915/selftests: exercise GPU access from the importer

Using PAGE_SIZE here potentially hides issues so bump that to something
larger. This should also make it possible for iommu to coalesce entries
for us. With that in place verify we can write from the GPU using the
importers sg_table, followed by checking that our writes match when read
from the CPU side.

v2: Switch over to igt_gpu_fill_dw(), which looks to be more widely
supported than the migrate stuff (at least OOTB).

References: https://gitlab.freedesktop.org/drm/intel/-/issues/7306


Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221028155029.494736-2-matthew.auld@intel.com