vga: Implement blinking of text cursor

Let the text cursor blink at 1.875 Hz, the original VGA cursor
frequency. No timer is used, instead we rely on the fact that the
display is updated periodically.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>