drm/i915/context: create & destroy ioctls

Add the interfaces to allow user space to create and destroy contexts.
Contexts are destroyed automatically if the file descriptor for the dri
device is closed.

Following convention as usual here causes checkpatch warnings.

v2: with is_initialized, no longer need to init at create
drop the context switch on create (daniel)

v3: Use interruptible lock (Chris)
return -ENODEV in !GEM case (Chris)

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