intel: Do the drawable x/y offset in intel_renderbuffer_map() for spans.

intel: Do the drawable x/y offset in intel_renderbuffer_map() for spans.

We were mapping the renderbuffer once, then walking over all the
buffers to map just the texture ones using the other texture mapping
function that handled the x/y offset to the image in the region.  But
then we would go and overwrite *those* mappings with the original
mappings for depth/stencil, which was wrong.

Instead, just walk over the attachments once and map the attachments.
Wasn't that easy?