display: Fix the resolution going to be tested
Gnome-control-center checks the display modes by cc_display_config_is_scaled_mode_valid() ... cc_display_config_dbus_is_scaled_mode_valid() to exclude unusable low resolutions.
However, it is the current using resolution that going to be tested by is_scaled_mode_allowed() in is_scale_allowed_by_active_monitors(), if it is global scaled required or configured as cloning mode originally. Therefor, it will check current using resolution again and again, instead of the enumerated one. This leads gnome-control-center building wrong resolution list on the panel.
This patch replaces the current mode with the enumerated mode to have the correct resolution to be tested by is_scaled_mode_allowed().
Fixes #903 (closed)