drm/rcar-du: dsi: Deduplicate mipi_dsi_pixel_format_to_bpp() usage

drm/rcar-du: dsi: Deduplicate mipi_dsi_pixel_format_to_bpp() usage

Call mipi_dsi_pixel_format_to_bpp() once in rcar_mipi_dsi_set_display_timing()
and store the value into a variable. This slightly simplifies the code.

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/20251028232959.109936-5-marek.vasut+renesas@mailbox.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>