drm/i915/bdw: Support 64b relocations

We don't actually return any to userspace yet, however we can pretend
like we do now so userspace will support it when it happens.

This is just to please Chris as the code itself isn't ready for > 64b
relocations.

v2: Rebase on top of the refactored relocate_entry_gtt|cpu functions.

v3: Squash in fixup from Rafal Barbalho for 64 byte relocs using cpu
relocs and those crossing a page boundary.

v4: Squash in a fixup for the fixup from Rafael.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Barbalho, Rafael <rafael.barbalho@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>