radeon: avoid possible divide by 0 in surface manager

Some users report hitting a divide by 0 with the tile split in
certain apps.  Tile_split shouldn't ever be 0 unless the surface
structure was not properly initialized.  I think there may be some
cases where mesa uses an improperly initialized surface struct,
but I haven't had time to track it down.

Bug:
https://bugs.freedesktop.org/show_bug.cgi?id=72425



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>