tdfxfb: make use of DDC information about connected monitor

Read DDC information from a connected monitor and use it to select initial
mode (if the mode is not specified).

Also, use the information to protect against modes outside the monitor
specs.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Tested-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>