i915/oa: Simplify updating contexts

We can remove the update-via-batch-buffer code path, which is basically an
effective duplicate of update-via-context-image path, if we notice that
after we have idled the GPU, we can update the context image even of the
kernel context directly. (Update-via-batch-buffer path existed only to
solve the problem of how to update the kernel context image.)

Only additional thing needed is to activate the edited configuration by
sending one empty request down the pipe. This accomplishes context restore
of the updated kernel context and so the OA configuration gets written out
to it's control registers.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180912152930.28237-1-tvrtko.ursulin@linux.intel.com