The existing util_get_narrow_range_coeffs doesn't work for RGB, since
all channels in RGB will share the same scale and bias.
Signed-off-by: Benjamin Cheng <benjamin.cheng@amd.com>
Reviewed-by: David Rosca <david.rosca@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41787>