drm/i915/selftests: remove duplicate include in mock_region.c

'drm/ttm/ttm_placement.h' included in
'drivers/gpu/drm/i915/selftests/mock_region.c' is duplicated.
It is also included on the 9 line.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Ran Jianping <ran.jianping@zte.com.cn>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211019090205.1003458-1-ran.jianping@zte.com.cn