drm/fb: Stop leaking physical address

For buffer sharing, use dma-buf instead. We can't set smem_start to 0
unconditionally since that's used by the fbdev mmap default
implementation. And we have plenty of userspace which would like to
keep that working.

This might break legit userspace - if it does we need to look at a
case-by-cases basis how to handle that. Worst case I expect overrides
for only specific drivers, since anything remotely modern should be
using dma-buf/prime now (which is about 7 years old now for DRM
drivers).

This issue was uncovered because Noralf's rework to implement a
generic fb_probe also implements it's own fb_mmap callback. Which
means smem_start didn't have to be set anymore, which blew up some
blob in userspace rather badly.

Acked-by: Sean Paul <[email protected]>
Cc: Gustavo Padovan <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: David Airlie <[email protected]>
Cc: John Stultz <[email protected]>
Cc: Noralf Trønnes <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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