drm/i915: Consolidate L3 remapping LRI

We can use a single MI_LOAD_REGISTER_IMM command packet to write all the
L3 remapping registers, shrinking the number of bytes required to emit
the context switch.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461833819-3991-9-git-send-email-chris@chris-wilson.co.uk