drm/i915/gem: Reduce ctx->engines_mutex for get_engines()

Take a snapshot of the ctx->engines, so we can avoid taking the
ctx->engines_mutex for a mere read in get_engines().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210114135612.13210-4-chris@chris-wilson.co.uk