vc1: Fix intensity compensation performance regression

Introduced by 28243b0d35b47bbf9abbd454fc444a6e0a9e7b71

Intensity compensation is always used once it was encountered, because
v->next_use_ic is never set back to zero.

Reset v->next_use_ic, when resetting v->next_luty/uv.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>