drm/rcar-du: dsi: Clean up VCLKSET register macros

drm/rcar-du: dsi: Clean up VCLKSET register macros

Introduce VCLKSET_BPP_MASK macro and use FIELD_PREP() to generate
appropriate bitfield from mask and value without bitshift. Remove
VCLKSET_COLOR_RGB which is never used, replace it with code comment.

Do not convert bits and bitfields to BIT() and GENMASK() yet, to be
consisten with the current style. Conversion to BIT() and GENMASK()
macros is done at the very end of this series in the last two patches.

Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Link: https://patch.msgid.link/20251028232959.109936-6-marek.vasut+renesas@mailbox.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>