drm/i915: Simplify error escape from cmdparser

We need to flush the destination buffer, even on error, to maintain
consistent cache state. Thereby removing the jump on error past the
clear, and reducing the loop-escape mechanism to a mere break.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191211110437.4082687-3-chris@chris-wilson.co.uk