fdo#40314 Force intercept only for linear/poly trendlines

Force intercept option is available only for linear and polynomial
trendlines

Change-Id: I98f551a51b4d1e85924ce23135588a03fabbea35
Reviewed-on: https://gerrit.libreoffice.org/7001
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>