drm/i915: Add missing POSTING_READ()s to BXT dbuf enable sequence

Do a POSTING_READ() between the DBUF_CTL register write and the
udelay() to make sure we really wait after the register write has
happened.

Spotted while reviewing Damien's SKL cdclk patch which had the
POSTING_READ()s.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>