agp/intel-gtt: Don't leak the scratch page

Recently discovered by enabling CONFIG_DMA_API_DEBUG in our CI. By the
looks of it broken since forever.

v2: Don't forget to set the scratch page back to wb (Chris). Reuse
intel_gtt_teardown_scratch_page for that (and fix it up to treat
needs_dmar y/n correctly).

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93793


Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1453901881-26425-1-git-send-email-daniel.vetter@ffwll.ch