drm/i915: remove unused has_dma_mapping flag

After the previous patch this flag will check always clear, as it's
never set for shmem backed and userptr objects, so we can remove it.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Yeah this isn't really fixes but it's a nice cleanup to
clarify the code but not really worth the hassle of backmerging. So
just add to -fixes, we're still early in -rc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>