drm/i915/gem: Unbind all current vma on changing cache-level

Avoid dangerous race handling of destroyed vma by unbinding all vma
instead. Unfortunately, this stops us from trying to be clever and only
doing the minimal change required, so on first use of scanout we may
encounter an annoying stall as it transitions to a new cache level.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112413


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202174310.2630302-1-chris@chris-wilson.co.uk