configure.ac: require xcb* for the omx/va/... when using x11 platform

Targets such as omx and va can work w/o anything X related. Mandate the
xcb* dependencies only when the X11 platform is selected.

Reported-by: Lukas Rusak <lorusak@gmail.com>
Fixes: 63e11ac2 ("configure: error out if building VA w/o supported
platform")
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Tested-by: Lukas Rusak <lorusak@gmail.com> (v1)
(cherry picked from commit 85a01723)