intel-gtt: initialize our own scratch page

The intel gtt fake agp driver is the only agp driver to use dma
address remapping. So it makes sense to fold this code back into the
only user (and thus reduce the reliance on the agp code).

This patch does the first step by initializing (and remapping) the
scratch page in a new function intel_gtt_setup_scratch_page.
Unfortunately intel_gtt_cleanup had to move to avoid a forward
declaration. The new scratch page is not yet used, though.

v2: Refactor out scratch page teardown.  Suggested by Chris Wilson on
irc. This makes it clear what's going on and results in a nice
symmetry between setup and teardown.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>