drm/i915: pch_irq_handler -> {ibx, cpt}_irq_handler

Cougar/Panther Point redefine the bits in SDEIIR pretty completely.
This function is just debugging, but if we're debugging we probably want
to be told accurate things instead of lies.

I'm told Lynx Point changes this yet more, but I have no idea how...

Note from Eugeni's review:

"For the record and for future enabling efforts, for LPT, bits 28-31
and 1-14 are gone since CPT/PPT (e.g., those must be zero). And there
is the bit 15 as a new addition, but we are not using it yet and
probably won't be using in foreseeable future."

Signed-off-by: Adam Jackson <ajax@redhat.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=35103
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>