drm/i915: Move i915_gem_object_phys_attach()

Prevent a forward declaration in the next patch.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170726181602.23527-1-chris@chris-wilson.co.uk


Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>