drm/vc4: Fix vblank timestamping for firmwarekms.

The core doesn't expect a false return from the scanoutpos function in
normal usage, so we were doing the precise vblank timestamping path
and thus "immediate" vblank disables (even though firmwarekms can't
actually disable vblanks interrupts, sigh), and the kernel would get
confused when getting timestamp info when also turning vblanks back
on.

Signed-off-by: Eric Anholt <eric@anholt.net>