drm/i915/selftests: Avoid choosing zero for phys_sz

Make sure we avoid ending up with a phys_sz of 0, or for phys_sz to be
larger than the actual size.

Closes: https://patchwork.freedesktop.org/series/73320/


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200212085432.1250807-1-chris@chris-wilson.co.uk