drm/ttm: Use the BIT macro for the TTM_TT_FLAGs

New code is recommended to use the BIT macro instead of the explicit
shifts. Change the older defines so that we can keep the style consistent
with upcoming changes.

v2:
- Also change the value of the _PRIV_POPULATED bit (Christian König)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230307144621.10748-4-thomas.hellstrom@linux.intel.com