drm/ttm: remove available_caching

Instead of letting TTM make an educated guess based on
some mask all drivers should just specify what caching
they want for their CPU mappings.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/390207/