egl/dri2: add image_loader_extension back into loader extensions for wayland

egl/dri2: add image_loader_extension back into loader extensions for wayland

before commit f871946594129500a67c05a6d9fe99db54b4bb64
image_loader_extension was always present in dri2_dpy->extensions,
after that commit it is only present for render nodes.

Its removal broke partial render based on buffer age on (at least)
raspberry pi.

Fixes: f8719465941 "egl/dri2: rework dri2_egl_display::extensions storage"
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>