sn: fix PRIME output support since xserver 1.20

Since "Make PixmapDirtyUpdateRec::src a DrawablePtr" in xserver, the
"src" pointer might point to the root window (created by the server)
instead of a pixmap (as created by xf86-video-intel). Use
get_drawable_pixmap to handle both cases.

When built with -fsanitize=address, the following test on a hybrid
graphics laptop will trigger a heap-buffer-overflow error due to
to_sna_from_pixmap receiving a window instead of a pixmap:

    xrandr --setprovideroutputsource modesetting Intel
    xrandr --output DP-1-1 --mode 2560x1440  # should not crash
    glxgears  # should display gears on both screens

With nouveau instead of modesetting, it does not crash but the external
monitor remains blank aside from a mouse cursor. This patch fixes both.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100086
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111976
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/sna_accel.c

@@ -17686,10 +17686,10 @@ static void sna_accel_post_damage(struct sna *sna)
 			continue;
 
 #ifdef HAS_DIRTYTRACKING_DRAWABLE_SRC
-		assert(dirty->src->type == DRAWABLE_PIXMAP);
+		src = get_drawable_pixmap(dirty->src);
+#else
+		src = dirty->src;
 #endif
-
-		src = (PixmapPtr)dirty->src;
 		dst = dirty->slave_dst->master_pixmap;
 
 		region.extents.x1 = dirty->x;