drm/sun4i: Make sunxi_rgb2yuv_coef constant

sunxi_rgb2yuv_coef is a table of RGB-to-YUV conversion coefficients.
They are programmed into the hardware, and can be declared constant.

Reported-by: Priit Laes <plaes@plaes.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>