drm/irq: Add drm_crtc_vblank_reset

At driver load we need to tell the vblank code about the state of the
pipes, so that the logic around reject vblank_get when the pipe is off
works correctly.

Thus far i915 used drm_vblank_off, but one of the side-effects of it
is that it also saves the vblank counter. And for that it calls down
into the ->get_vblank_counter hook. Which isn't really a good idea
when the pipe is off for a few reasons:
- With runtime pm the register might not respond.
- If the pipe is off some datastructures might not be around or
  unitialized.

The later is what blew up on gen3: We look at intel_crtc->config to
compute the vblank counter, and for a disabled pipe at boot-up that's
just not there. Thus far this was papered over by a check for
intel_crtc->active, but I want to get rid of that (since it's fairly
race, vblank hooks are called from all kinds of places).

So prep for that by adding a _reset functions which only does what we
really need to be done at driver load: Mark the vblank pipe as off,
but don't do any vblank counter saving or event flushing - neither of
that is required.

v2: Clarify the code flow slightly as suggested by Ville.

v3: Fix kerneldoc spelling, spotted by Laurent.

Cc: Ville Syrjälä <[email protected]>
Cc: Laurent Pinchart <[email protected]>
Cc: Imre Deak <[email protected]>
Reviewed-by: Imre Deak <[email protected]> (v2)
Acked-by: Dave Airlie <[email protected]>
Acked-by: Laurent Pinchart <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>

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