drm/fsl-dcu: use mode flags for hsync/vsync polarity

The current default configuration is as follows:
- Invert VSYNC signal (active LOW)
- Invert HSYNC signal (active LOW)

The mode flags allow to specify the required polarity per
mode. Furthermore, none of the current driver settings is
actually a standard polarity.

This patch applies the current driver default polarities as
explicit flags to the display which has been introduced with
the driver (NEC WQVGA "nec,nl4827hc19-05b"). The driver now
also parses the flags field and applies the configuration
accordingly, by using the following values as standard
polarities: (e.g. when no flags are specified):
- VSYNC signal not inverted (active HIGH)
- HSYNC signal not inverted (active HIGH)

Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>