The sg_table for our backing store might contain addresses from
stolen-memory or in the future local-memory, at which point this is no
longer a dma-iterator. As a consequence we should now break on NULL
iter.sgp, instead of dmap == 0 which is considered an invalid dma
address.

As a bonus, gcc much prefers this construct,

  Function                                     old     new   delta
  gen8_ggtt_insert_entries                     211     192     -19
  gen6_ggtt_insert_entries                     292     262     -30
  i915_error_object_create                     996     954     -42
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190829201919.21493-1-matthew.auld@intel.com