drm/i915: Warn if irq_mask isn't ~0 during vlv/cvh display irq postinstall

We expect vlv_display_irq_reset() to have been called prior to
vlv_display_irq_postinstall() so let's WARN if that isn't the case.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460382992-28728-8-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>