drm/i915/gt: Expose engine properties via sysfs

Preliminary stub to add engines underneath /sys/class/drm/cardN/, so
that we can expose properties on each engine to the sysadmin.

To start with we have basic analogues of the i915_query ioctl so that we
can pretty print engine discovery from the shell, and flesh out the
directory structure. Later we will add writeable sysadmin properties such
as per-engine timeout controls.

An example tree of the engine properties on Braswell:
/sys/class/drm/card0
└── engine
   ├── bcs0
   │   ├── capabilities
   │   ├── class
   │   ├── instance
   │   ├── known_capabilities
   │   └── name
   ├── rcs0
   │   ├── capabilities
   │   ├── class
   │   ├── instance
   │   ├── known_capabilities
   │   └── name
   ├── vcs0
   │   ├── capabilities
   │   ├── class
   │   ├── instance
   │   ├── known_capabilities
   │   └── name
   └── vecs0
      ├── capabilities
   ├── class
   ├── instance
      ├── known_capabilities
   └── name

v2: Include stringified capabilities
v3: Include all known capabilities for futureproofing.
v4: Combine the two caps loops into one

v5: Hide underneath Kconfig.unstable for wider discussion

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Tested-by: Steve Carbonari <steven.carbonari@intel.com>
Reviewed-by: Steve Carbonari <steven.carbonari@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228131716.3243616-1-chris@chris-wilson.co.uk