wlr_output: do not modeset to current mode
Created by: RedSoxFan
Supersedes #1545 which was reverted in #1550 due to swaywm/sway#3685, which is a sway issue and has swaywm/sway#3704 open to fix it.
Fixes swaywm/sway#3659
There is no point in modesetting an output to a mode that it is already set to. Modesetting will cause the output to briefly flicker which is undesirable for a noop. This prevents modesetting any wlr_output, regardless of the backend, to it's currently set mode.