intel: Fix software detiling of system stencil buffers

intel: Fix software detiling of system stencil buffers

If a window system stencil buffer had a region with odd height, then the
calculated y offset needed for software detiling was off by one.  The bug
existed in intel_{map,unmap}_renderbuffer_s8() and in the intel_span.c
accessors.

Fixes the following Piglit tests on gen7:
    general/depthstencil-default_fb-readpixels-24_8
    general/depthstencil-default_fb-readpixels-FLOAT-and-USHORT

Fixes SIGABRT in the following Piglit tests on gen7:
    general/depthstencil-default_fb-blit
    general/depthstencil-default_fb-copypixels
    general/depthstencil-default_fb-drawpixels-24_8
    general/depthstencil-default_fb-drawpixels-FLOAT-and-USHORT

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>