drm/i915: Clear execbuf's vma backpointer upon release

drm/i915: Clear execbuf's vma backpointer upon release

commit 96ef520d65ef ("drm/i915: Eliminate lots of iterations over the
execobjects array") jiggled around the error handling and replace a test
that we cleaned up properly after ourselves with an assertion. That
assertion failed because in the release function (moments after the
assertion) we were indeed forgetting to mark the vma as cleared. The
consequence was when testing an invalid relocation address, we would try
to release the vma twice (following the couple of attempts to verify the
address) and on the second release notice that the first release was
incomplete.

Testcase: igt/gem_reloc_overflow/invalid-address
Fixes: 96ef520d65ef ("drm/i915: Eliminate lots of iterations over the execobjects array")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170622104722.2583-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>