intel: Simplify stencil detiling arithmetic

intel: Simplify stencil detiling arithmetic

When calculating the y offset needed for detiling window system stencil
buffers, replace the term
   region->height * 2 + region->height % 2 - 1
with
   rb->Height - 1 .

The two terms are incidentally equivalent due to some out-of-date,
incorrect code in the Intel DRI2 glue for DDX. (See
intel_process_dri2_buffer_with_separate_stencil(), line ``buffer_height /=
2;``).

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