drm/i915: Reduce nested prepare_remote_context() to a trylock

On context retiring, we may invoke the kernel_context to unpin this
context. Elsewhere, we may use the kernel_context to modify this
context. This currently leads to an AB-BA lock inversion, so we need to
back-off from the contended lock, and repeat.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111732
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: a9877da2

 ("drm/i915/oa: Reconfigure contexts on the fly")
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191126065521.2331017-1-chris@chris-wilson.co.uk