drm/tegra: dpaux: Fix transfers larger than 4 bytes

The DPAUX read/write FIFO registers aren't sequential in the register
space, causing transfers larger than 4 bytes to cause accesses to non-
existing FIFO registers.

Fixes: 6b6b6042 ("drm/tegra: Add eDP support")
Signed-off-by: Thierry Reding <treding@nvidia.com>