drm/omap: fix missing scaler pixel fmt limitations

OMAP2 and OMAP3/AM4 have limitations with the scaler:
- OMAP2 can only scale XRGB8888
- OMAP3/AM4 can only scale XRGB8888, RGB565, YUYV and UYVY

The driver doesn't check these limitations, which leads to sync-lost
floods.

This patch adds a check for the pixel formats when scaling.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190930103840.18970-5-tomi.valkeinen@ti.com