drm/nouveau/drm/nouveau/clk: fix tstate to pstate calculation

According to the tstate calculation in nvkm_clk_tstate(),
the range of tstate is from -(clk->state_nr - 1) to 0,
it mean the tstate is negative value. But in nvkm_pstate_work(),
it use (clk->state_nr - 1 - clk->tstate) to limit pstate,
it's not correct.
This patch fix it to use (clk->state_nr - 1 + clk->tstate) to
limit pstate.

Signed-off-by: Wei Ni <wni@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>