intel: Share the implementation of BO unmap between CPU and GTT mappings.

Before this, consumers of the libdrm API that might map a buffer
either way had to track which way was chosen at map time to call the
appropriate unmap.  This relaxes that requirement by making
drm_intel_bo_unmap() always appropriate.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>