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>