how to interpret of "pvclust" dendrogram and finding height for cutting dendrogram?
You do not need to give links to your images, you can embed images in your questions using the "image" icon.
FPKM (and RPKM) might not be the best RNA-seq normalization method, see this and this.


pvclust providing p-values for the clusters computed by hclust. To do so, bootstrap resampling is used. From their documentation: 


  Values on the edges of the clustering are p-values (%). Red values are
  AU p-values, and green values are BP values. Clusters with AU larger
  than 95% are highlighted by rectangles, which are strongly supported
  by data.


Basically you are getting a clustering pattern and a statistic associated with each of the clusters giving you an idea about the "statistical significance" of the clusters you have obtained (i.e. clusters most probably not being a result of strong batch effetcs, ...).

To be able to comment on the resulting clusters making sense or not, you will need to consult somebody with domain knowledge, it is not a number or two that would determine the quality of the output but also how good is the fit between your result and the (expected) biology. For example in your case with TCGA data:


Do the samples cluster by batch?
Do tumor and adjacent normal tissue samples aggregate into different clusters?
Do the samples in the same cluster share the same mutation/methylation pattern/disease subgroup?