drm/i915: Set up VLV_MASTER_IER consistently

We're lacking VLV_MASTER_IER setup from valleyview_irq_preinstall(), so
add it there. Also cargo cult in some POSTING_READ()s to match the other
platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460571598-24452-3-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>