drm: Don't return 0 for a value used as a denominator

Static analysis will be unhappy if a function can theoretically return
0 and we're trying to divide by that value.

Mark that case that cannot occur as a BUG() instead.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>