drm: rcar-du: lvds: Move LVDS enable code to separate code section

To prepare for a rework of the LVDS disable code, which will need to be
called from rcar_lvds_pclk_disable(), move the LVDS enable code,
currently stored in the __rcar_lvds_atomic_enable() function, to a
separate code section separate from bridge operations. It will be then
extended with the LVDS disable code.

As part of this rework the __rcar_lvds_atomic_enable() function is
renamed to rcar_lvds_enable() to more clearly indicate its purpose.

No functional change intended.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>