randr: Skip workspaces which are assigned to a different output when creating a new...

randr: Skip workspaces which are assigned to a different output when creating a new workspace

If you had workspace 1, 2, 3, 4 on LVDS1 and you enabled HDMI2 (where workspace
1 to workspace 5 are assigned to HDMI2), i3 would look for a new workspace for
LVDS1 (since all workspaces were moved), create workspace 5, move that over due
to assignment and then create workspace 6. Effectively, you would end up with
an empty workspace 5.