drm/omap: dsi: fix and cleanup ddr_clk_always_on

The driver ignores MIPI_DSI_CLOCK_NON_CONTINUOUS, and always uses
non-continuous clock.

Fix this by using MIPI_DSI_CLOCK_NON_CONTINUOUS and at the same time,
drop ddr_clk_always_on field which seems pretty useless.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-82-tomi.valkeinen@ti.com