drm/i915: Nuke posting reads from plane update/disable funcs

No need for the posting reads in the plane update/disable hooks.
If we need a posting read for something then a single one at the
very end would be sufficient. We have that anyway in the form
of eg. scanline/frame counter reads.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181101150605.18235-2-ville.syrjala@linux.intel.com


Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>