Since gen3 can't handle >1024 wide sources with vertical scaling
let's not advertize such modes in the mode list. Less tempetation
to the user to try out things that won't work.

v2: s/IS_GEN3(dev_priv/IS_GEN(dev_priv, 3)/
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181112170000.27531-17-ville.syrjala@linux.intel.comReviewed-by: Imre Deak <imre.deak@intel.com>