drm/i915/gtt: Use shallow dma pages for scratch

We only use the dma pages for scratch, and so do not need to allocate
the extra storage for the shadow page directory.

v2: Refrain from reintroducing I915_PDES

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712075818.20616-1-chris@chris-wilson.co.uk