drm/i915/bdw: Make gen8_gmch_probe

Probing gen8 is similar to gen6. To make the code cleaner and more
maintainable however we can use the probe functions to split it out.

v2: Rebased on top of update gtt probe infrastructure.

v3: Rebased on top of Kenneth' Graunke's ->pte_encode refactoring.

V4: Resolve conflicts with Ben's latest ppgtt patches, also switch to
gen < 8 testing instead of gen <= 7.

v5: Resolve conflicts with address space vfunc changes in upstream.

v6: Use 39b DMA mask. At least, for this mode, it is the correct mask.
(Imre)

Cc: Imre Deak <imre.deak@intel.com>
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>