drm: Only update final vblank count when precise ts is available

drm: Only update final vblank count when precise ts is available

Drivers without a hardware vblank counter simply can't account for the
vblanks that happened while the vblank interrupt was off. To check
this grab a vblank timestamp and if the result is dubious follow the
normal save-and-disable logic.

Drivers should prevent this by setting vblank_disable_allowed = false,
but since running vblank interrupts constantly is not good for power
consumption most drivers lie. Testing for precise vblank timestamps is
the next best thing we can check for.

Suggested-by: Mario Kleiner <[email protected]>
Cc: Mario Kleiner <[email protected]>
Cc: Matt Roper <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Reviewed-by: Mario Kleiner <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>