patch 9.0.1729: screenpos() wrong when w_skipcol and cpoptions+=n

Problem:    screenpos() wrong result with w_skipcol and cpoptions+=n
Solution:   Use adjust_plines_for_skipcol() instead of subtracting
            w_skipcol.

closes: #12625

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
diff --git a/src/drawscreen.c b/src/drawscreen.c
index 86fec15..90e1984 100644
--- a/src/drawscreen.c
+++ b/src/drawscreen.c
@@ -1792,7 +1792,8 @@
 		j = wp->w_lines[0].wl_lnum - wp->w_topline;
 	    if (j < wp->w_height - 2)		// not too far off
 	    {
-		i = plines_m_win(wp, wp->w_topline, wp->w_lines[0].wl_lnum - 1);
+		i = plines_m_win(wp, wp->w_topline, wp->w_lines[0].wl_lnum - 1,
+									 TRUE);
 #ifdef FEAT_DIFF
 		// insert extra lines for previously invisible filler lines
 		if (wp->w_lines[0].wl_lnum != wp->w_topline)