drm/radeon/kms: add some loop timeouts in pageflip code

Avoid infinite loops waiting for surface updates if a GPU
reset happens while waiting for a page flip.

See:
https://bugs.freedesktop.org/show_bug.cgi?id=43191



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@kernel.org
Reviewed-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Tested-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>