drm/i915: switch to default context on idle

To keep things as sane as possible, switch to the default context before
idling. This should help free context objects, as well as put things in
a more well defined state before suspending.

v2: remove seqno from context switch call (daniel)
return error on failed context switch instead of WARN+continue (daniel)

v3: move idling to i915_gpu idle (from i915_gem_idle) (Chris)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>