drm/i915/migrate: move the sanity check

Move the sanity check that both src and dst are never both system
memory, which should never happen on discrete, and likely means we have
a bug. The only exception is on integrated where we trigger this path in
the selftests.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220324172143.377104-2-matthew.auld@intel.com