drm/tilcdc: Remove obsolete "ti,tilcdc,slave" dts binding support

This patch removes DRM_TILCDC_SLAVE_COMPAT option for supporting the
obsolete "ti,tilcdc,slave" device tree binding. The new of_graph based
binding - that is widely used in other drm driver too - has been
supported since Linux v4.2. Maintaining the the backwards dts
conversion code in the DRM_TILCDC_SLAVE_COMPAT has become a nuisance
for the device/of development so the we decided to drop it after Linux
v4.14, the 2017 LTS.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Rob Herring <robh@kernel.org>