drm/i915: add a selftest for the mmio_bases table

Check that the entries are in reverse gen order and that all entries
with gen > 0 have an mmio base set.

v2: loop forward, simplify logic, use i915_subtests (Chris)

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180314182653.26981-2-daniele.ceraolospurio@intel.com