drm/i915: Improve fallback ring waiting

A few improvements to the fallback method for waiting upon ring space:

1. Fix the start/end wait tracepoints to always be paired.
2. Increase responsiveness of checking
3. Mark the process as waiting upon io
4. Check for signal interruptions

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Brad Volkin <bradley.d.volkin@intel.com>
[danvet: Drop the s/msleep/io_schedule_timeout/ change again since the
latter isn't exported.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>