drm/i915/gt: Use the kernel_context to measure the breadcrumb size

We set up a dummy ring in order to measure the size we require for our
breadcrumb emission, so that we don't have to manually count dwords! We
can pass in the kernel_context to use for this so that if required it is
known for the breadcrumb emitter, and we can reuse some details from the
kernel_context to reduce the number of temporaries we have to mock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200207125827.2787472-1-chris@chris-wilson.co.uk