drm/exynos: dp: Remove support for unused dptx-phy

Now that we have moved to generic phy based bindings,
we don't need to have any code related to older dptx-phy.
Nobody is using this dptx-phy anymore, so removing the
same.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>