egl/dri2: set WL_bind_wayland_display in a consistent way

egl/dri2: set WL_bind_wayland_display in a consistent way

Introduce a helper and use it throughout the platform code. This allows
us to reduce the amount of ifdef(s) and (potentially) use
kms_swrast_dri.so for !drm platforms (namely wayland and x11).

Note: in the future as other platforms (android, surfaceless) support
the extension they can reuse the helper.

v2: Rebase, check for device_name.
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>