gpu: nvgpu: Use sw ops for PMU PG unit

Some functions are not accessing hardware directly
but are being called using HAL ops: For example

g->ops.pmu.pmu_elpg_statistics,
g->ops.pmu.pmu_pg_init_param,
g->ops.pmu.pmu_pg_supported_engines_list,
g->ops.pmu.pmu_pg_engines_feature_list,
g->ops.pmu.pmu_is_lpwr_feature_supported,
g->ops.pmu.pmu_lpwr_enable_pg,
g->ops.pmu.pmu_lpwr_disable_pg,
g->ops.pmu.pmu_pg_param_post_init,
g->ops.pmu.save_zbc

Change the function access by using sw ops, like:
Create new functions:

int nvgpu_pmu_elpg_statistics(struct gk20a *g, u32 pg_engine_id,
		struct pmu_pg_stats_data *pg_stat_data);
void nvgpu_pmu_save_zbc(struct gk20a *g, u32 entries);
bool nvgpu_pmu_is_lpwr_feature_supported(struct gk20a *g,
			u32 feature_id);

JIRA NVGPU-3209

Change-Id: I6db9b43c7c4a5054720a72487302b740b091044d
Signed-off-by: Divya Singhatwaria <dsinghatwari@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/2110963
Reviewed-by: svc-mobile-coverity <svc-mobile-coverity@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Mahantesh Kumbar <mkumbar@nvidia.com>
Reviewed-by: Vaibhav Kachore <vkachore@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/common/gr/zbc.c

@@ -134,7 +134,7 @@ static int nvgpu_gr_zbc_add(struct gk20a *g, struct nvgpu_gr_zbc *zbc,
 		entries = max(zbc->max_used_color_index,
 					zbc->max_used_depth_index);
 		if (g->elpg_enabled) {
-			g->ops.pmu.save_zbc(g, entries);
+			nvgpu_pmu_save_zbc(g, entries);
 		}
 	}