drm/omap: change default signal polarities and drives

If the given videomode does not specify DISPLAY_FLAG_* for the specific
signal property, the driver used a default value. These defaults were
never thought through, as the expectation was that all the DISPLAY_FLAGS
are always set explicitly.

With DRM bridge and panel drivers this is not the case, and while that
issue should be resolved in the future, it's still good to have sane
signal defaults.

This patch changes the defaults to what the hardware has as reset
defaults. Also, based on my experience, I think they make sense and are
more likely correct than the defaults without this patch.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200417114151.25843-1-tomi.valkeinen@ti.com
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>