drm/mipi-dsi: Drop MIPI_DSI_MODE_VSYNC_FLUSH flag

Drop the unused MIPI_DSI_MODE_VSYNC_FLUSH flag. Whether or not a display
FIFO flush on vsync is required to avoid sending garbage to the panel is
not a property of the DSI link, but of the integration between display
controller and DSI host bridge.

Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250627-dsi-vsync-flush-v2-4-4066899a5608@pengutronix.de


Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>