mesa: don't map depth+stencil buffer twice in glReadPixels()

mesa: don't map depth+stencil buffer twice in glReadPixels()

In slow_read_depth_stencil_pixels_separate() we might have separate
depth and stencil buffers or a combined buffer.  In the later case,
don't map the buffer twice.  This function is used when the depth
scale/bias pixel transfer values are not the defaults.

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=42963

Reviewed-by: José Fonseca <jfonseca@vmware.com>