v3d: Enable perpendicular line caps when line smoothing

v3d: Enable perpendicular line caps when line smoothing

V3D has a bit to set the line caps to be perpendicular to the line
rather than aligned to the edges of the framebuffer. I don’t know what
the disadvantages are of enabling this, but I noticed by experimentation
that enabling line smoothing on the Intel driver also enables nicer line
caps, so it seems nice to enable it here too.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5624>