drm/i915: Cleanup request skip decision

Since we now only skip banned contexts, preventing the skip of default
contexts is no longer sensible. For a similar argument as before
'commit 7ec73b7e ("drm/i915: Only skip requests once a context is banned")'
we end up with an inconsistent API if we only mark future execbufs from
the default ctx as banned but fail to mark those currently executing
as failed.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484668747-9120-3-git-send-email-mika.kuoppala@intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>