drm: Extract&Document drm_irq.h

The drm_irq docs want one function from drmP.h, but that one is a
serious mess. Extract it, and while at it improve the docs a bit.
There's a bit a header loop issue since core data structures like
drm_device and drm_driver aren't in their own headers yet, which means
the drm_irq.h include in drmP.h needs to be in just the right spot :(

Also noticed that drm_vblank_crtc->last_wait is entirely unused,
remove it.

v2: git add drm_irq.h ...

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

Documentation/gpu/drm-kms.rst

@@ -649,5 +649,5 @@ Vertical Blanking and Interrupt Handling Functions Reference
 .. kernel-doc:: drivers/gpu/drm/drm_irq.c
    :export:
 
-.. kernel-doc:: include/drm/drmP.h
-   :functions: drm_crtc_vblank_waitqueue
+.. kernel-doc:: include/drm/drm_irq.h
+   :internal: