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 <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Acked-by: Rodrigo Vivi <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Tested-by: Steve Carbonari <[email protected]>
Reviewed-by: Steve Carbonari <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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