drm/i915: Remove lrc default desc from GEM context

We only compute the lrc_descriptor() on pinning the context, i.e.
infrequently, so we do not benefit from storing the template as the
addressing mode is also fixed for the lifetime of the intel_context.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Prathap Kumar Valsan <prathap.kumar.valsan@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730133035.1977-9-chris@chris-wilson.co.uk