drm/radeon: make 64bit fences more robust v3

Only increase the higher 32bits if we really detect a wrap around.

v2: instead of increasing the higher 32bits just use the higher
    32bits from the last emitted fence.
v3: also use last emitted fence value as upper limit.

The intention of this patch is to make fences as robust as
they where before introducing 64bit fences. This is
necessary because on older systems it looks like the fence
value gets corrupted on initialization.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=51344

Should also fix:
https://bugs.freedesktop.org/show_bug.cgi?id=54129
https://bugs.freedesktop.org/show_bug.cgi?id=54662
https://bugzilla.redhat.com/show_bug.cgi?id=846505
https://bugzilla.redhat.com/show_bug.cgi?id=845639

3.5 needs a separate patch due to changes in the
fence code.  Will send that out separately.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>