weston-keyboard: Create input_panel_surface earlier

If we only create the input_panel_surface after we've already created
the window and tried to get some content for it, then we're never going
to enter the input_panel_surface committed handler, so we'll never get
the chance to properly map the surface.

Follow the other surface types by creating the input-panel surface
before we try to attach anything to it.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Fixes: bdf2019e ("desktop-shell: Map input panel surfaces before views")