xilinx: drm: dp: Update with new helpers

Update the DP driver to use the new drm DP helpers as the old helpers
are being moved away. Replace use of i2c structures with
struct drm_dp_aux and helper functions.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>