wayland-drm: static inline wayland_drm_buffer_get

wayland-drm: static inline wayland_drm_buffer_get

The function is effectively a direct function call into
libwayland-server.so.

Thus GBM no longer depends on the wayland-drm static library, making the
build more straight forward. And the resulting binary is a bit smaller.

Note: we need to move struct wayland_drm_callbacks further up,
otherwise we'll get an error since the type is incomplete.

v2: Rebase, beef-up commit message, update meson, move struct
wayland_drm_callbacks.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com> (v1)
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> # meson bit only
Acked-by: Eric Engestrom <eric.engestrom@imgtec.com> # for the rest
Reviewed-by: Dylan Baker <dylan@pnwbakers.com> # meson