drm/i915/gem: Disallow bonding of virtual engines (v3)

This adds a bunch of complexity which the media driver has never
actually used.  The media driver does technically bond a balanced engine
to another engine but the balanced engine only has one engine in the
sibling set.  This doesn't actually result in a virtual engine.

This functionality was originally added to handle cases where we may
have more than two video engines and media might want to load-balance
their bonded submits by, for instance, submitting to a balanced vcs0-1
as the primary and then vcs2-3 as the secondary.  However, no such
hardware has shipped thus far and, if we ever want to enable such
use-cases in the future, we'll use the up-and-coming parallel submit API
which targets GuC submission.

This makes I915_CONTEXT_ENGINES_EXT_BOND a total no-op.  We leave the
validation code in place in case we ever decide we want to do something
interesting with the bonding information.

v2 (Jason Ekstrand):
 - Don't delete quite as much code.

v3 (Tvrtko Ursulin):
 - Add some history to the commit message

Signed-off-by: Jason Ekstrand <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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