drm/i915/gtt: Move scratch_pd and scratch_pt into vm struct

Scratch page is part of struct i915_address_space. Move other
scratch entities into the same struct. This is a preparatory patch
for having only one instance of each scratch_pt/pd.

v2: make commit msg more readable

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com> (v1)
[danvet: Bikeshed summary to avoid confusion with vmas.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>