docs: dma-buf: fix some warnings

Fix those warnings:

	Documentation/driver-api/dma-buf.rst:182: WARNING: Title underline too short.

	Indefinite DMA Fences
	~~~~~~~~~~~~~~~~~~~~

	Documentation/driver-api/dma-buf.rst:88: WARNING: Unknown target name: "fence poll support".

The first one is due to a shorter markup. The second one is
because the chapter name was wrong.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/b2bc0bc88eb913635cfece13cc9f6eff7668d333.1599660067.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>