video: dw_mipi_dsi: Fix hsync/vsync settings

These must be read from timings->flags, like other DSI HOST drivers do.

And they must not be inverted either. Low means low.

Without this fix, panel drivers that set *SYNC_LOW produce corrupted
output on screen (shifted horizontally and vertically by back porch
distance).

Signed-off-by: Ondrej Jirman <megi@xff.cz>