Some more VC4 vblank fixes and timestamping improvements.

[Mario Kleiner]

Hi Eric and all,

this batch prevents setting modes one shouldn't set,
adds precise vblank timestamping for interlaced video
modes, and one fix for vblank en/disable during crtc
en/disable.

All successfully tested, also with timing measurement equipment,
on the RPi 2B.

Thanks,
-mario