ivi-shell: introduce get_weston_view

ivi-shell: introduce get_weston_view

The internal API "get_weston_view" is introduced, which returns the
weston_view of the given ivi_layout_surface. The API returns a NULL
pointer, if the ivi_layout_surface does not have any weston_view.

The weston_view is required in many places of ivi-shell implementation.
Therefore, this API will reduce lines of code. Furthermore, it will
increase the maintainability of the ivi-shell implementation.

Old way of getting the weston_view was flawed, because the views list of
the weston_surface is read without controlling the existence of the
weston_surface.  New implementation explicitly throws an error if the
weston_surface does not exist.

Signed-off-by: Emre Ucan <eucan@de.adit-jv.com>
Tested-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
Reviewed-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
[Pekka: Line-wrapped the commit message.]