drm: Make wait_event_interruptible interruptible and fix its return value.

* __wait_event_common now works like wait_event_interruptible_timeout does
  in Linux.

* wait_event_interruptible uses different return values than the other
  wait_event variants.

* translate -ERESTARTSYS to -EINTR in two obvious places in radeon_gem.c,
  before returning from an ioctl handler.