drm: drm_atomic.c: Adjust end of block comment

Add a new line with */ on the last line of a block comment to follow the
Linux kernel coding conventions.
Problem found by checkpatch.

Signed-off-by: Beatriz Martins de Carvalho <martinsdecarvalhobeatriz@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/387b6b852a72d6a98133cead5e4d0e3c39dd5094.1618756333.git.martinsdecarvalhobeatriz@gmail.com