mx3fb: avoid screen flash when panning 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 custom fb_set_par() method and then calls custom
fb_pan_display() method. The current implementation of mx3fb
reinitializes IPU display controller every time the custom
fb_set_par() method is called, which makes the screen flash
if fb_set_var() is called to do panning frequently. This patch
compares the new var info with the cached old one to decide
whether we really need to reinitialize IPU display controller.
We ignore xoffset and yoffset update because it doesn't require
to reinitialize the controller. Users may specify activate field
of var info with FB_ACTIVATE_NOW and FB_ACTIVATE_FORCE to
reinialize the controller by force.

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