drm/i915: Use locked access to ctx->engines in set_priority

This essentially reverts

commit 89ff76bf
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Apr 2 13:42:18 2020 +0100

    drm/i915/gem: Utilize rcu iteration of context engines

Note that the other use of __context_engines_await have disappeard in
the following commits:

ccbc1b97 ("drm/i915/gem: Don't allow changing the VM on running contexts (v4)")
c7a71fc8 ("drm/i915: Drop getparam support for I915_CONTEXT_PARAM_ENGINES")
4a766ae4 ("drm/i915: Drop the CONTEXT_CLONE API (v2)")

None of these have any business to optimize their engine lookup with
rcu, unless extremely convincing benchmark data and a solid analysis
why we can't make that workload (whatever it is that does) faster with
a proper design fix.

Also since there's only one caller of context_apply_all left and it's
really just a loop, inline it and then inline the lopp body too. This
is how all other callers that take the engine lock loop over engines,
it's much simpler.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210810130523.1972031-1-daniel.vetter@ffwll.ch