drm/ttm: set the tt caching state at creation time

All drivers can determine the tt caching state at creation time,
no need to do this on the fly during every validation.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://patchwork.freedesktop.org/patch/394253/