efifb: patch to allow userspace to unbind efi framebuffer driver from VGA device

This patch allows userspace to direct efifb to let go of the VGA device 
and unregister it's framebuffer. As far as I can tell, the Linux kernel 
framebuffer console knows to let go when efifb unregisters it's 
framebuffer. The problem I'm trying to solve is that I need efifb so the 
kernel can show it's status on-screen during boot, but then I need efifb 
to step aside and let a better driver load and take the VGA device later 
during boot up.

The custom Linux distribution I've made for myself uses a userspace 
program to "boot" secondary VGA devices and load both fbdev and drm/kms 
drivers to bring video online. When I wrote this, I needed the ability 
for efifb to let go so that it could load bochsfb to better use the VGA 
output of VirtualBox and bochs. Without it, my console is stuck at 
whatever video mode the UEFI BIOS happened to leave it at and the more 
specialized driver cannot acquire the resources it needs to do it's work.

The patch adds a sysfs device file "bind_fb" to the 
/sys/bus/platform/devices/efifb.0 filesystem tree. Writing "1" causes it 
to re-register the framebuffer, "0" to unregister. Checks are in place 
to not register the framebuffer if already registered, etc.

I realize it's not perfect and I wanted to know what I could do to 
improve it and eventually make it to mainline.

On a related issue, when will efifb make use of the Graphics Output 
Protocol through UEFI to offer modesetting? Is that possible? If no 
specialized drivers are available it'd be nice to at least have basic 
modesetting as needed.