drm/mode: add the GTF algorithm in kernel space

Add the GTF algorithm in kernel space. And this function can be called to
generate the required modeline.

I copied it from the file of xserver/hw/xfree86/modes/xf86gtf.c. What I have
done is to translate it by using integer calculation. This is to avoid
the float-point calculation in kernel space.
At the same tie I also refer to the function of fb_get_mode in
drivers/video/fbmon.c

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>