drm: omapdrm: dpi: Replace OMAP SoC model checks with DSS model

The DPI code only needs to differentiate between major OMAP revisions,
which can be obtained from the DSS compatible string. Replace the OMAP
SoC model checks to prepare for removal of the OMAP SoC version platform
data.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>