drm: Remove unused fbdev_list members

I noticed that intel_fbdev->our_mode is unused. Introduced by
79e53945 ("DRM: i915: add mode setting support").

Then I noticed that intel_fbdev->fbdev_list is unused as well.
Introduced by 38651674 ("drm/fb: fix fbdev object model +
cleanup properly.") in i915, nouveau and radeon.

Subsequently cargo culted to amdgpu, ast, cirrus, qxl, udl,
virtio and mgag200.

Already removed from the latter with cc59487a ("drm/mgag200:
'fbdev_list' in 'struct mga_fbdev' is not used").

Remove it from the others.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>