Enter bootloader command should turn off display(s)

[rpavlik]

Since we're otherwise disabling normal functioning (I believe), it seems wise to disable displays on entering the bootloader, and probably also disable any other interrupts (such as videoinput interrupts). This may require a new interface method on VideoInput (shutdown) which would disable interrupts, if applicable, and perhaps also disable hotplug detection if possible/applicable.

Noticed when entering bootloader on HDK2 left an image on the screen.