avoid uninitialised attr in rasops_scrollback()

[Jonathan Gray]

Don't use attr uninitialised.  Avoids glitches seen when using
scrollback with radeondrm.

Index: rasops.c
===================================================================
RCS file: /cvs/src/sys/dev/rasops/rasops.c,v
retrieving revision 1.53
diff -u -p -r1.53 rasops.c
--- rasops.c    27 Apr 2018 21:36:12 -0000      1.53
+++ rasops.c    29 Apr 2018 06:34:19 -0000
@@ -1927,6 +1927,7 @@ rasops_scrollback(void *v, void *cookie,
                return;
 
        rasops_cursor(ri, 0, 0, 0);
+       ri->ri_alloc_attr(ri, 0, 0, 0, &attr);
        ri->ri_eraserows(ri, 0, ri->ri_rows, attr);
        for (row = 0; row < ri->ri_rows; row++) {
                for (col = 0; col < ri->ri_cols; col++) {