drm/vblank: drop use of DRM_WAIT_ON()

DRM_WAIT_ON() is from the deprecated drm_os_linux header and
the modern replacement is the wait_event_*.

The return values differ, so a conversion is needed to
keep the original interface towards userspace.
Introduced a switch/case to make code obvious.

Analysis from Michel Dänzer:

The waiting condition rely on all relevant places where vblank_count
is modified calls wake_up(&vblank->queue).

drm_handle_vblank():
- Calls wake_up(&vblank->queue)

drm_vblank_enable():
- There is no need here because there can be no sleeping waiters
  in the queue, because vblank->enabled == false immediately
  terminates any waits.

drm_crtc_accurate_vblank_count():
- This is called from interrupt handlers, at least from
  amdgpu_dm.c:dm_pflip_high_irq(). Not sure it needs to wake up
  the queue though, the driver should call
  drm_(crtc_)_handle_vblank anyway.

drm_vblank_disable_and_save():
- It can be called from an interrupt, via drm_handle_vblank ->
  vblank_disable_fn. However, the only place where
  drm_vblank_disable_and_save can be called with sleeping waiters
  in the queue is in drm_crtc_vblank_off, which wakes up the queue
  afterwards (which terminates all waits, because
  vblank->enabled == false at this point).

v3:
- Added analysis to changelog from Michel Dänzer
- Moved return result handling inside if (req_seq != seq) (Daniel V)
- Reused more of the former logic - resulting in simpler code
- Dropped Reviewed-by from Sean Paul as this is a new implementation

v2:
- Fix so the case where req_seq equals seq was handled properly
- quick hack to check if IGT became happy
- Only sent to igt, not to dri-devel

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190726210658.GA6299@ravnborg.org