radv: Only require DRM 3.23.

Turns out kernel 4.15 only goes up to amdgpu 3.23 . 3.35 is way
too new. Too new for e.g. ChromeOS.

Fixes: 1df4f11e ("radv: require DRM 3.35+")
Closes: mesa/mesa#4728


Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <mesa/mesa!10576>