drm: use seqlock for vblank time/count

This patch aims to replace the roll-your-own seqlock implementation with
full-blown seqlock'. We also remove the timestamp ring-buffer in favour
of single timestamp/count pair protected by a seqlock. In turn this
means we can now increment the vblank freely without the need for
clamping.

v2:
  - reduce the scope of the seqlock, keeping vblank_time_lock
  - make the seqlock per vblank_crtc, so multiple readers aren't blocked by
    the writer

Cc: Mario Kleiner <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Signed-off-by: Matthew Auld <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Reviewed-by: Mario Kleiner <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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