[x86] remove uses of magic macros for boot_params access

Instead of using magic macros for boot_params access, simply use the
boot_params structure.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
index e9afb7e..a569871 100644
--- a/drivers/video/console/vgacon.c
+++ b/drivers/video/console/vgacon.c
@@ -372,7 +372,7 @@
 #endif
 	}
 
-	/* SCREEN_INFO initialized? */
+	/* boot_params.screen_info initialized? */
 	if ((ORIG_VIDEO_MODE  == 0) &&
 	    (ORIG_VIDEO_LINES == 0) &&
 	    (ORIG_VIDEO_COLS  == 0))