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>