drm/omap: fix analog tv-out modecheck

omapdrm rejects all venc (analog tv-out) videomodes, due to somewhat
strict checking of the values, making tv-out unusable.

We only support two videomodes, one for PAL and one for NTSC, so instead
of trying to check every field in the videomode struct, this patch makes
the driver check only the pixel clock and the size of the display.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>