drm/omap: dsi: skip dsi_vc_enable_hs when already in correct mode

Simplify and optimize dsi_vc_enable_hs() so that it can be called
without checking the current HS/LP mode. Make dsi_vc_enable_hs() return
if the VC is already in the correct mode.

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/20201215104657.802264-69-tomi.valkeinen@ti.com