drm/rcar-du: dsi: Document TXVMSETR PIXWDTH as bitfield

drm/rcar-du: dsi: Document TXVMSETR PIXWDTH as bitfield

The register TXVMSETR bitfield PIXWDTH is not a single bit, but a
bitfield. Add a MASK macro and document that the only allowed value
that can ever be written into the bitfield is the current value, 1.
No functional change.

Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Link: https://patch.msgid.link/20251028232959.109936-3-marek.vasut+renesas@mailbox.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>