[media] mtk-vcodec: add HAS_DMA dependency

This fixes this kbuild test robot error:

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   329f4152911c276b074bec75a0443f88821afdb7
commit: c1023ba74fc77dc56dc317bd98f5060aab889ac1 [media] drivers/media/platform/Kconfig: fix VIDEO_MEDIATEK_VCODEC dependency
config: m32r-allyesconfig (attached as .config)
compiler: m32r-linux-gcc (GCC) 4.9.0
reproduce:
        wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git/plain/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        git checkout c1023ba74fc77dc56dc317bd98f5060aab889ac1
        # save the attached .config to linux build tree
        make.cross ARCH=m32r

All errors (new ones prefixed by >>):

   drivers/media/v4l2-core/videobuf2-dma-contig.c: In function 'vb2_dc_get_userptr':
>> >> drivers/media/v4l2-core/videobuf2-dma-contig.c:486:2: error: implicit declaration of function 'dma_get_cache_alignment' [-Werror=implicit-function-declaration]
     unsigned long dma_align = dma_get_cache_alignment();
     ^
   cc1: some warnings being treated as errors

This driver depends on HAS_DMA for dma_get_cache_alignment().

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>