drm/omapdrm: Remove unused struct csc_coef_rgb2yuv

After commit 64ff1891, struct csc_coef_rgb2yuv is not used any more
and can be removed as well.

Fixes: 64ff1891 ("drm/omap: Enable COLOR_ENCODING and COLOR_RANGE properties for planes")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221014024810.21675-1-yuancan@huawei.com