savagefb: VGA state save and restore

Allow the saving and restoration of VGA text mode.  The state is saved on the
first open and restored on the last close. Because of the VGA registers are
linearly mapped to the MMIO space, MMIO access is used which is not limited to
X86 platforms nor to the primary display device.

An echo 0 > /sys/class/vtconsole/vtcon1/bind will convert the display from
graphics to text mode.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>