drm/i915: Make ironlake_{enable,disable}_display_irq() static inlines

drm/i915: Make ironlake_{enable,disable}_display_irq() static inlines

ironlake_{enable,disable}_display_irq() each just call
ilk_update_display_irq() so let's make them static inlines.

While at it s/ironlake/ilk/ to make things shorter, and a bit more
consistent with the ibx functions.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448294777-13722-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>