drm/irq: Add kms-native crtc interface functions

We need to start somewhere ... With this the only places left in i915
where we use pipe integers is in the interrupt handling code. And
there it actually makes some amount of sense.

v2:
- Polish kerneldoc a bit (Thierry).
- Drop "dev" parameter since it's unecessary.
- Split out i915 changes (Thierry).

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