drm/ttm: drop unusued function declaration

This was removed in
f5a9a938

 ("drm/ttm: remove TTM_MEMTYPE_FLAG_CMA")

but the the declaration was left dangling.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728045129.21065-1-airlied@gmail.com