media: MAINTAINERS: Make Tomasz the main maintainer of videobuf2

Tomasz is the most active member from the people listed currently in the
MAINTAINERS file, but is currently listed as a reviewer. Change the
entry into a maintainer and move to the top of the list.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>