drisw: Fix and simplify drawable setup

drisw: Fix and simplify drawable setup

We don't want to require a visual for the drawable, because there exist
fbconfigs that don't correspond to any visual (say a 565 pixmap|pbuffer
config on a depth-24 display). Fortunately, we don't need one either.
Passing the visual to XCreateImage serves only to fill in the XImage's
{red,green,blue}_mask fields, which libX11 itself never uses, they exist
only for the client's convenience, and we don't care. And we already
have the drawable depth in glx_config::rgbBits. So replace the
XVisualInfo field in the drawable private with a pointer to the
glx_config.

Having done that driswCreateGCs becomes trivial, so inline it into its
caller.

Gitlab: https://gitlab.freedesktop.org/mesa/mesa/issues/1194
Reviewed-by: Eric Anholt <eric@anholt.net>