drisw: Fix invalid pointer arithmetic

Use of void * in pointer arithmetic is illegal, use char * instead.
Fixes: cf54bd5e83 ("drisw: use shared memory when possible")

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>

src/gallium/winsys/sw/dri/dri_sw_winsys.c

@@ -233,7 +233,7 @@ dri_sw_displaytarget_display(struct sw_winsys *ws,
    unsigned width, height, x = 0, y = 0;
    unsigned blsize = util_format_get_blocksize(dri_sw_dt->format);
    unsigned offset = 0;
-   void *data = dri_sw_dt->data;
+   char *data = dri_sw_dt->data;
 
    /* Set the width to 'stride / cpp'.
     *