tests/gem_pipe_control_store_loop: Delete bogus assertion

tests/gem_pipe_control_store_loop: Delete bogus assertion

target_bo->offset was just being used to verify that the batch
submission worked and fortunately was not being relied upon for any
subsequent conditions. However, address 0 is valid and so the assertion
itself was bogus as it is possible (almost assured with full-ppgtt) for
the target_bo to be located at address 0.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72984
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>