i965/gen7: Minify blit size for stencil tree copy
i965/gen7: Minify blit size for stencil tree copy

Found by the piglit 'fbo-depth-array stencil-clear' test when
implementing blorp blit splitting for gen7.

Signed-off-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>
Reviewed-by: Ben Widawsky <73675debcd8a436be48ec22211dcf44fe0df0a64@bwidawsk.net>
Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler