drm/omap: dsi: enable HS before sending the frame

We currently use a single VC for sending commands and pixel data. The
LP/HS mode for pixel data is correctly set to HS by accident, as we have
set the VC to HS already earlier.

However, if we use a different VC for video data, the VC is in LP mode.
Fix this by always enabling HS mode before starting a frame update.

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/20201215104657.802264-70-tomi.valkeinen@ti.com