vga: add vga_scanline_invalidated helper

Add vga_scanline_invalidated helper to check whenever a scanline was
invalidated.  Add a sanity check to fix OOB read access for display
heights larger than 2048.

Only cirrus uses this, for hardware cursor rendering, so having this
work properly for the first 2048 scanlines only shouldn't be a problem
as the cirrus can't handle large resolutions anyway.  Also changing the
invalidated_y_table size would break live migration.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170421091632.30900-4-kraxel@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>