drm/i915: Unref context on failed eb_create

I opted to do this instead of grabbing the context reference after
eb_create since eb_create can potentially call the shrinker, and that
makes things very complicated. This simple patch balances the ref count
without requiring a great deal of review to make sure the shrinker path
is safe.

Theoretically (by design) the shrinker can end up destroying a context,
which enforces the reasoning for doing the fix this way instead of
moving the reference to later in the function.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>