output: auto-select mode when enabling output by client request

output: auto-select mode when enabling output by client request

If e.g. kanshi requests us to enable an output but does not specify the
mode, and the output has not previously been modeset, then the width +
height + refresh rate will all be zero. In this case, we should select
the best mode just as when auto-configuring a new output.

We need to do the mode auto-selection in both the test & commit stages
for consistency, so factor out a new output_test_auto() function to help
with this.

Remove a guard against a refresh rate of zero, which was a workaround
for wlroots 0.17 and is no longer needed -- it is incompatible with mode
auto-selection since the refresh rate is zero in that case.