drm/i915: Clear VLV_MASTER_IER around irq processing

Like on CHV, let's clear out the master irq enable bit when we ack
GT/PM interrupts. This will allow GT/PM interrupts to re-raise the
CPU interrupt if we fail to clear all the bits from the IIR(s).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460571598-24452-5-git-send-email-ville.syrjala@linux.intel.com


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