drm/i915: Factor out ilk_update_display_irq()

Extract the core of ironlake_{enable,disable}_display_irq() into a new
function. We'll have further use for it later.

v2: Warn about invalid mask vs. enable bits (Paulo)

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>