Change drm_bo_type_dc to drm_bo_type_device and comment usage of this value.

I couldn't figure out what drm_bo_type_dc was for; Dave Airlie finally clued
me in that it was the 'normal' buffer objects with kernel allocated pages
that could be mmapped from the drm device file.

I thought that 'drm_bo_type_device' was a more descriptive name.

I also added a bunch of comments describing the use of the type enum values and
the functions that use them.