drm/ttm: optimize bo_kmap_type values

A micro-optimization on the function ttm_kmap_obj_virtual().

By defining the values of enum ttm_bo_kmap_obj::bo_kmap_type to have a
bit indicating iomem, size of the function ttm_kmap_obj_virtual() will be
reduced by 16 bytes on x86_64 (gcc 4.1.2).

ttm_kmap_obj_virtual() may be heavily used, when buffer objects are
accessed via wrappers, that work for both kinds of memory addresses:
iomem cookies and kernel virtual.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Dave Airlie <airlied@redhat.com>