drm/nouveau: fix pramdac_table range checking

get_tmds_index_reg reads some value from stack when mlv happens
to be equal to size of pramdac_table array. Fix it.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>