drm/omap: HDMI5: Add interlace support

Add the missing bits for interlace:

* Set VBLANK_OSC if the videomode's vblank is fractional
* Halve the vertical timings for interlace
* Double the horizontal timings for double-pixel mode
* Set FC_PRCONF properly for double-pixel mode

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