drm/i915: bikeshed the pipe CRC irq functions a bit

- Give them an _irq_handler postfix, like all the other irq stuff.
- Shuffle the DEBUG_FS=n dummy functions around a bit. This is prep
  work to extract all the crc debug stuff into intel_display_testing.c

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