drm/i915: Drop useless posting reads from cursor commit

There should be no need to do posting reads between all the cursor
register accessess. Let's just drop them.

v2: Rebase due to I915_WRITE_FW() and uncore.lock

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Link: http://patchwork.freedesktop.org/patch/msgid/20170327185546.2977-8-ville.syrjala@linux.intel.com