intel: Fix separate stencil in builtin DRI2 backend

intel: Fix separate stencil in builtin DRI2 backend

intelAllocateBuffer() was oblivious to separate stencil buffers.  This
patch fixes it to allocate a non-tiled stencil buffer with special pitch,
just as the DDX does.

Without this, any app that attempted to create an EGL surface with stencil
bits would crash. Of course, this affected only environments that used the
builtin DRI2 backend, such as Android and Wayland.

Fixes GLBenchmark2.1 on Android on gen7.

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