intel: Fix region dimensions for stencil buffers received from DDX

intel: Fix region dimensions for stencil buffers received from DDX

I changed the dimensions of the stencil buffer's region, as allocated by
the DDX, at xf86-video-intel commit
   commit 3e55f3e88b40471706d5cd45c4df4010f8675c75
   dri: Do not tile stencil buffer
But I forgot to make the analogous update to the Intel DRI2 glue in Mesa.
This patch makes that update.

Surprisingly, the mismatch did not cause any bugs. But the mismatch, if
left unfixed, *would* create bugs in the next commit.

Note: This is a candidate for the 7.11 branch.
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>