drm/i915: L3 cache remapping is part of context switching

Move the i915_gem_l3_remap function such that it next to the context
switching, which is where we perform the L3 remap.

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/1461833819-3991-8-git-send-email-chris@chris-wilson.co.uk