drm: do not modeset to current mode
Created by: RedSoxFan
Fixes swaywm/sway#3659 Supersedes swaywm/sway#3161
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 returns early in drm_connector_set_mode
when attempting to modeset to the current mode.