radv: Invert condition for all samples identical during resolve.

the samples_identical instruction returns 0 if they are differet, so
we have to do the extra work if the result is 0, not if it is != 0.

Fixes: f4e499ec "radv: add initial non-conformant radv vulkan driver"
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit c39947ce)