i965/miptree: Rework create flags

i965/miptree: Rework create flags

The only one of the three remaining flags that has anything whatsoever
to do with layout is TILING_NONE.  This commit renames them to
MIPTREE_CREATE_*, documents the meaning of each flag, and makes the
create functions take an actual enum type so GDB will print them nicely.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>