drm/i915: Remove surplus POSTING_READs before wait_for_vblank

... as wait_for_vblank (and friends) will do a flush of the MMIO writes
anyway.

References: https://bugs.freedesktop.org/show_bug.cgi?id=34601


Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>