intel/blorp: Fix the early return condition in convert_to_single_slice

intel/blorp: Fix the early return condition in convert_to_single_slice

The convert_to_single_slice operation is *mostly* idempotent.  The only
non-repeatable thing it does is that, when it sets the intratile offset
fields, it just overwrites them instead of doing a += operation.  This is
supposed to be ok because we have an early return at the top that should
make it bail of the surface is already a single slice.  Unfortunately, the
if condition has been broken ever since it was first added in 96fa98c18.
This commit fixes the condition and adds an assert to ensure we don't stomp
any non-zero intratile offsets.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>