drm/i915/execlists: Weak parallel submission support for execlists

A weak implementation of parallel submission (multi-bb execbuf IOCTL) for
execlists. Doing as little as possible to support this interface for
execlists - basically just passing submit fences between each request
generated and virtual engines are not allowed. This is on par with what
is there for the existing (hopefully soon deprecated) bonding interface.

We perma-pin these execlists contexts to align with GuC implementation.

v2:
 (John Harrison)
  - Drop siblings array as num_siblings must be 1
v3:
 (John Harrison)
  - Drop single submission
v4:
 (John Harrison)
  - Actually drop single submission
  - Use IS_ERR check on return value from intel_context_create
  - Set last request to NULL on unpin

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211222223532.28698-1-matthew.brost@intel.com