drm/nouveau/bios/therm: handle vbioses with duplicate entries (mostly nva5)

Some vbioses have extra useless entries after "the end" of the table. This is
problematic since all of the vbios I found with this issue redefine the
pwm freq divider to insane levels (52750 Hz instead of 2500), thus breaking
fan management.

The first solution to solve this mess would be to change the length of the
table. The solution I choose was simply to avoid setting the pwm freq twice
as the other redefinitions are harmless with our current parser.

Signed-off-by: Martin Peres <martin.peres@labri.fr>
Reported-by: Mariusz Bialonczyk <manio@skyboo.net>
Tested-by: Mariusz Bialonczyk <manio@skyboo.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>