drm/mediatek: set DMA max segment size

This driver requires imported PRIME buffers to appear contiguously in
its IO address space. Make sure this is the case by setting the maximum
DMA segment size to a more suitable value than the default 64KB.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>