i915: Don't emit waits for pending flips before emitting synchronous flips.

i915: Don't emit waits for pending flips before emitting synchronous flips.

The assumption is that synchronous flips are not isolated usually, and waiting
for all of them could result in stalling the pipeline for long periods of time.

Also use i915_emit_mi_flush() instead of an old-fashioned way to achieve the
same effect.