drm/nouveau/clk: Use list_for_each_entry_from_reverse

It's better to use "list_for_each_entry_from_reverse" for iterating list
than "for loop" as it makes the code more clear to read.
This patch replace "for loop" with "list_for_each_entry_from_reverse"
and "start" variable with "cstate" which helps in refactoring
the code and also "cstate" variable is more commonly used in the other
functions.

changes in v2:
"start" variable is removed, before "cstate" variable was removed
but "cstate" is more common so preferred "cstate" over "start".
Signed-off-by: Arushi Singhal <arushisinghal19971997@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

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

@@ -109,18 +109,17 @@ nvkm_cstate_valid(struct nvkm_clk *clk, struct nvkm_cstate *cstate,
 
 static struct nvkm_cstate *
 nvkm_cstate_find_best(struct nvkm_clk *clk, struct nvkm_pstate *pstate,
-		      struct nvkm_cstate *start)
+		      struct nvkm_cstate *cstate)
 {
 	struct nvkm_device *device = clk->subdev.device;
 	struct nvkm_volt *volt = device->volt;
-	struct nvkm_cstate *cstate;
 	int max_volt;
 
-	if (!pstate || !start)
+	if (!pstate || !cstate)
 		return NULL;
 
 	if (!volt)
-		return start;
+		return cstate;
 
 	max_volt = volt->max_uv;
 	if (volt->max0_id != 0xff)
@@ -133,8 +132,7 @@ nvkm_cstate_find_best(struct nvkm_clk *clk, struct nvkm_pstate *pstate,
 		max_volt = min(max_volt,
 			       nvkm_volt_map(volt, volt->max2_id, clk->temp));
 
-	for (cstate = start; &cstate->head != &pstate->list;
-	     cstate = list_prev_entry(cstate, head)) {
+	list_for_each_entry_from_reverse(cstate, &pstate->list, head) {
 		if (nvkm_cstate_valid(clk, cstate, max_volt, clk->temp))
 			break;
 	}