drm/i915: Make ibx_{enable,disable}_display_interrupt() static inlines

No reason why ibx_{enable,disable}_display_interrupt() couldn't be
static inlines instead of cpp macros.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448294777-13722-2-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>