drm: omapdrm: hdmi: Replace OMAP SoC model check with HDMI xmit version

The HDMI wrapper code only needs to differentiate between major OMAP
revisions, which can be obtained from the HDMI transmitter 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>