The pipe might already have been shut down, and then it's not a good
idea to call hw accessor functions. Instead use the same logic as
drm_vblank_off which has all the necessary checks to avoid troubles or
inconsistency.

Noticed by Imre while reviewing my patches to remove some sanity
checks from ->get_vblank_counter.

v2: Try harder. disable_and_save can still access the vblank stuff
when vblank->enabled isn't set. It has to, since vlbank irq could be
disable but the pipe is still on when being called from
drm_vblank_off. But we still want to use that code for more code
sharing. So add a check for vblank->enabled on top - if that's not set
we shouldn't have anyone waiting for the vblank. If we have that's a
pretty serious bug.

The other issue that Imre spotted is drm_vblank_cleanup. That code
again calls disable_and_save and so suffers from the same issues. But
really drm_irq_uninstall should have cleaned that all up, so replace
the code with WARN_ON. Note that we can't delete the timer cleanup
since drivers aren't required to use drm_irq_install/uninstall, but
can do their own irq handling.

v3: Make it clear that all that gunk in drm_irq_uninstall is really
just bandaids for UMS races between the irq/vblank code. In UMS
userspace is in control of enabling/disabling interrupts in general
and vblanks specifically.

v4: Imre observed that KMS drivers all call drm_vblank_cleanup before
drm_irq_uninstall (as they should), so again the code in there is dead
for KMS (due to dev->num_crtcs == 0 after drm_vblank_cleanup). Or
should be, so only WARN for KMS - with UMS userspace could try to do
evil things.

v5: After more discussion on irc we've gone back to v3: the
del_timer_sync is required in all cases in drm_vblank_cleanup, but
let's restrict the WARN_ON to kms drivers only. Imre was also
concerned that bad things could happen without the disable_and_save
call. But we immediately free vblank structures afterwards which makes
the save useless. And drm_handle_vblank has a check for dev->num_crtcs
to avoid surprises with ums.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>