MAINTAINERS: Better regex for dma_buf|fence|resv

We're getting some random other stuff that we're not really interested
in, so match only word boundaries. Also avoid the capture group while
at it.

Suggested by Joe Perches.

Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Cc: Joe Perches <joe@perches.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200317205643.1028398-1-daniel.vetter@ffwll.ch