drm/i915/gt: Expose engine->mmio_base via sysfs

Use the per-engine sysfs directory to let userspace discover the
mmio_base of each engine. Prior to recent generations, the user
accessible registers on each engine are at a fixed offset relative to
each engine -- but require absolute addressing. As the absolute address
depends on the actual physical engine, this is not always possible to
determine from userspace (for example icl may expose vcs1 or vcs2 as the
second vcs engine). Make this easy for userspace to discover by
providing the mmio_base in sysfs.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Steve Carbonari <steven.carbonari@intel.com>
Tested-by: Steve Carbonari <steven.carbonari@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228131716.3243616-2-chris@chris-wilson.co.uk