drm/i915: Mark all default contexts as uninitialised after context loss

When the GPU is reset or state lost through suspend, every default
legacy context needs to reload their state - both the golden render
state and the L3 mapping. Only context images explicitly saved to memory
(i.e. all execlists and non-default legacy contexts) will retain their
state across the reset.

v2: Rebase

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466776558-21516-4-git-send-email-chris@chris-wilson.co.uk