drm/i915: Purge loose pages if we run out of DMA remap space

If the DMA remap fails, one cause can be that we have too many objects
pinned in a small remapping table, such as swiotlb. (DMA remapping does
not trigger the shrinker by itself on its normal failure paths.)  So try
purging all other objects (using i915_gem_shrink_all(), sparing our own
pages as we have yet to assign them to the obj->pages) and try again. If
there are no pages to reclaim (and consequently no pages to unmap), the
shrinker will report 0 and we fail with -ENOSPC as before.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170106152240.5793-2-chris@chris-wilson.co.uk