pvr: fix infinite recursion in pvr_cmd_buffer_{start,end}_sub_cmd
requested to merge luigi.santivetti/mesa:staging/deqp-api-clear-depth-stencil-attachment-single-layer-d16-unorm-64x11_upstream into main
/cc @frankbinns @CreativeCylon @MTCoster
Reported-by: James Glanville james.glanville@imgtec.com