drm: avoid division by 0 in drm_pflip_cb with bad drivers

Seems like some drivers only increment msc every other page flip when
running in interlaced mode (I'm looking at you nouveau). I.e. it seems
to be incremented at the frame rate, rather than the field rate.
Obviously we can't work with this, so shame the driver and bail.

On intel this isn't an issue, as msc is incremented at field rate
there.

This means presentation feedback won't work correctly in interlaced
modes with those drivers, but who in their right mind uses an
interlaced mode these days, anyway?

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