drm/amd/display: Fix race.

Problem :
A race between two adjecent page flips makes the earlier one
to release an alocated frame buffer for the subsequent one -
since there are 2 frambuffer swapped back and forth between flips,
the 'new' fb of the later flip is actually the 'previous' fb for the earlier flip.
Fix:
Don't set fb->address = 0 in cleanup hook, this is unnecessery and
erases the newly cached adress that was set in prepare_fb of the second
flip.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>