wayland-drm: avoid deprecated use of struct wl_resource

wayland-drm: avoid deprecated use of struct wl_resource

Wayland v1.2 with commit 1488c96a5db ("Add accessor functions for
wl_resource and deprecate wl_client_add_resource") paves the way towards
making wl_resource opaque.

Namely, new helpers were introduced and the struct was annotated as
deprecated.

Since wayland headers are normally installed in /usr/include, which is
in -isystem, GCC did not generate warnings as documented in the manual.
  "Warnings from system headers are normally suppressed..."

Signed-off-by: Micah Fedke <micah.fedke@collabora.co.uk>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
[Emil Velikov: add commit message]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>