For tracking the MSC when the present can be executed separately from
the target MSC.

Allows removing the requeue field instead, plus more later.

v2:
* Rename wait_msc → exec_msc (Roman Gilg)
* Use exec_msc = target_msc instead of exec_msc++, for clarity.
* Bug fix: Set exec_msc = target_msc also if present_flip returned
  false in present_execute.
v3:
* Set exec_msc = target_msc also if present_wnmd_flip returned
  false in present_wnmd_execute, for consistency.
v4:
* Specifically check for exec_msc == crtc_msc + 1 in
  present_execute_wait/copy, to avoid re-introducing
  https://bugs.freedesktop.org/show_bug.cgi?id=94596

 .

Reviewed-by: Roman Gilg <subdiff@gmail.com>
Tested-by: Roman Gilg <subdiff@gmail.com>