compositor-drm: Allow weston_output_mode_switch_to_native() to work.

compositor-drm: Allow weston_output_mode_switch_to_native() to work.

Initialize output->native_mode with the initially chosen
mode for an output, so weston_output_mode_switch_to_native()
has something to work with and can switch back from temporary
selected modes to the outputs native mode. Before, this was a
no-op.

This allows an output to switch back to its default mode if
a former toplevel fullscreen shell surface created via method
WL_SHELL_SURFACE_FULLSCREEN_METHOD_DRIVER gets destroyed, or
it gets demoted to non-fullscreen, or if modesetting on the
output failed for some reason.

v2: Modified and split into a separate patch from original
    patch "Allow restore_output_mode() to work properly.",
    as suggested by Derek Foreman.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Derek Foreman <derekf@osg.samsung.com>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>