Take bo type argument out of the ioctl interface.

The buffer object type is still tracked internally, but it is no longer
part of the user space visible ioctl interface.  If the bo create ioctl
specifies a non-NULL buffer address we assume drm_bo_type_user,
otherwise drm_bo_type_dc.  Kernel side allocations call
drm_buffer_object_create() directly and can still specify drm_bo_type_kernel.
Not 100% this makes sense either, but with this patch, the buffer type
is no longer exported and we can clean up the internals later on.