backends/drm: fix some things with KScreen configs
It can happen that a config sets all outputs to be disabled, or that the primary output is disabled. In those cases, do not apply the config. For when the config can't be applied but we need to enable some output, explicitly enable an output.
There was also an issue with configs having the refresh rate saved as a floating point number, which got wrongly rounded down by an implicit cast. In order to work around that, explicitly round the value. We'll also want to port KScreen to use mHz instead, to prevent such issues from happening in the future.
Edited by Xaver Hugl