drm/i915/selftests: handle more rounding

Ensure we account for potential rounding up of lmem objects.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/5485


Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220406173023.1039107-1-matthew.auld@intel.com