fbdev: fbcon: check if mode can handle new screen

Check if the mode can properly display the screen.  This will be needed by
drivers where the capability is not constant with each mode.  The function
fb_set_var() will query fbcon the requirement, then it will query the driver
(via a new hook fb_get_caps()) its capability.  If the driver's capability
cannot handle fbcon's requirement, then fb_set_var() will fail.

For example, if a particular driver supports 2 modes where:

mode1 = can only display 8x16 bitmaps
mode2 = can display any bitmap

then if current mode = mode2 and current font = 12x22

fbset <mode1> /* mode1 cannot handle 12x22 */
fbset will fail

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>