drm/i915: Tune down debug output when context is banned

If we have stopped rings then we know that test is running
so no need for spam. In addition, only spam when default
context gets banned.

v2: - make sure default context ban gets shown (Chris)
    - use helper for checking for default context, everywhere (Chris)

v3: - dont be quiet when debug is set (Ben, Daniel)

Reference: https://bugs.freedesktop.org/show_bug.cgi?id=73652


Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>