drm/ttm: Make ttm_bo_mem_compat available

There are cases where it is desired to see if a proposed placement
is compatible with a buffer object before calling ttm_bo_validate().

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Cc: <stable@vger.kernel.org>
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This is the first of a 3-patch series to fix a black screen
issue observed on Ubuntu 16.04 server.