drm/i915/selftests: mock test io_size

Check that mappable vs non-mappable matches our expectations.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220225145502.331818-7-matthew.auld@intel.com