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

drivers/gpu/drm/nouveau/nvkm/subdev/clk/base.c

@@ -214,7 +214,7 @@ nvkm_pstate_work(struct work_struct *work)
 	pstate = clk->pwrsrc ? clk->ustate_ac : clk->ustate_dc;
 	if (clk->state_nr && pstate != -1) {
 		pstate = (pstate < 0) ? clk->astate : pstate;
-		pstate = min(pstate, clk->state_nr - 1 - clk->tstate);
+		pstate = min(pstate, clk->state_nr - 1 + clk->tstate);
 		pstate = max(pstate, clk->dstate);
 	} else {
 		pstate = clk->pstate = -1;