drm/i915: use enum pipe consistently in i915_irq.c

Request by Ville in his review of the CRC stuff. This converts
everything but ilk_display_irq_handler since that needs a bit more
than a simple search&replace to look nice.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>