gl-renderer: Ensure gl_buffer_state is present for direct-display

This patch makes sure we have a gl_buffer_state present when using
direct-display protocol extensions (which forbids any GL imports, and
assumes a direct path with the display unit to perform a KMS import).

Without this patch we would basically have no gl_buffer_state at repaint
time because we never manged to create one, as direct-display code path
will return much early.

Partially fixes gitlab.freedesktop.org/wayland/weston/-/issues/621.

Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>