The GTT and PPGTT can be thought of more generally as GPU address
spaces. Many of their actions (insert entries), state (LRU lists), and
many of their characteristics (size) can be shared. Do that.

The change itself doesn't actually impact most of the VMA/VM rework
coming up, it just fits in with the grand scheme of abstracting the GPU
VM operations. GGTT will usually be a special case where we either know
an object must be in the GGTT (dislay engine, workarounds, etc.).

The scratch page is left as part of the VM (even though it's currently
shared with the ppgtt code) because in the future when we have Full
PPGTT, I intend to create a separate scratch page for each.

v2: Drop usage of i915_gtt_vm (Daniel)
Make cleanup also part of the parent class (Ben)
Modified commit msg
Rebased

v3: Properly share scratch page (Imre)
Finish commit message (Daniel, Imre)
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>