media: dvb: fix DVB_MMAP dependency

Enabling CONFIG_DVB_MMAP without CONFIG_VIDEOBUF2_VMALLOC results
in a link error:

drivers/media/dvb-core/dvb_vb2.o: In function `_stop_streaming':
dvb_vb2.c:(.text+0x894): undefined reference to `vb2_buffer_done'
drivers/media/dvb-core/dvb_vb2.o: In function `dvb_vb2_init':
dvb_vb2.c:(.text+0xbec): undefined reference to `vb2_vmalloc_memops'
dvb_vb2.c:(.text+0xc4c): undefined reference to `vb2_core_queue_init'
drivers/media/dvb-core/dvb_vb2.o: In function `dvb_vb2_release':
dvb_vb2.c:(.text+0xe14): undefined reference to `vb2_core_queue_release'
drivers/media/dvb-core/dvb_vb2.o: In function `dvb_vb2_stream_on':
dvb_vb2.c:(.text+0xeb8): undefined reference to `vb2_core_streamon'
drivers/media/dvb-core/dvb_vb2.o: In function `dvb_vb2_stream_off':
dvb_vb2.c:(.text+0xfe8): undefined reference to `vb2_core_streamoff'
drivers/media/dvb-core/dvb_vb2.o: In function `dvb_vb2_fill_buffer':
dvb_vb2.c:(.text+0x13ec): undefined reference to `vb2_plane_vaddr'
dvb_vb2.c:(.text+0x149c): undefined reference to `vb2_buffer_done'

This adds a 'select' statement for it, plus a dependency that
ensures that videobuf2 in turn works, as it in turn depends on
VIDEO_V4L2 to link, and that must not be a module if videobuf2
is built-in.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>