drm/i915: fix another use-after-free in i915_gem_evict_everything

Also here, i915_gem_evict_vm causes an unbind, which can end up dropping
the last ref to the ppgtt.

Triggered by igt gem_evict_everything test.

Testcase: igt/gem_evict_everything
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Chris Wilson <chris@cris-wilsonc.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>