v3dv/uniforms: update VIEWPORT_X/Y_SCALE uniforms for v71

v3dv/uniforms: update VIEWPORT_X/Y_SCALE uniforms for v71

As the packet CLIPPER_XY scaling, this needs to be computed on 1/64ths
of pixel, instead of 1/256ths of pixels.

As this is the usual values that we get from macros, we add manually a
v42 and v71 macro, and define a new helper (V3DV_X) to get the value
for the current hw version.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25450>