mx3fb: support pan display with fb_set_var

Users may call FBIOPUT_VSCREENINFO ioctrl to do pan display.
This ioctrl relies on fb_set_var() to do the job. fb_set_var()
calls the custom fb_set_par() method and then calls the custom
fb_pan_display() method. Before calling the custom fb_pan_display()
method, info->var is already updated from the new *var in fb_set_var().
And, the custom fb_pan_display() method checks if xoffset and yoffset
in info->var and the new *var are different before doing actual panning,
which prevents the panning from happening within fb_set_var() context.
This patch caches the current var info locally in mx3fb driver so that
pan display with fb_set_var is supported.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>