egl/wayland: introduce dri2_wl_add_configs_for_visuals() helper

egl/wayland: introduce dri2_wl_add_configs_for_visuals() helper

Analogous to previous commits - with an extra bonus.

Current code, apart from not attributing the lack of 'per visual'
and overall configs also overwrites the newly added config.

Namely if the dpy supports two or more of the supported formats
(XRGB8888, ARGB8888 and RGB565) earlier configs will be overwritten
and the the final one will be stored, since the we use the same index
for all three in our dri2_add_config call.

v2: Use correct comparison in loop conditional (Eric)
    Use valid C initializer (Gurchetan)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>