drm/i915: Flush outstanding requests before allocating new seqno

In very rare cases (such as a memory failure stress test) it is possible
to fill the entire ring without emitting a request. Under this
circumstance, the outstanding request is flushed and waited upon. After
space on the ring is cleared, we return to emitting the new command -
except that we just cleared the seqno allocated for this operation and
trigger the sanity check that a request is only ever emitted with a
valid seqno. The fix is to rearrange the code to make sure the
allocation of the seqno for this operation is after any required flushes
of outstanding operations.

The bug exists since the preallocation was introduced in
commit 9d773091
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Nov 27 16:22:52 2012 +0000

    drm/i915: Preallocate next seqno before touching the ring

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>