MLK-11920 video: fbdev: mxc ipuv3 fb: Use fb_set_var() to unblank a framebuffer

MLK-11920 video: fbdev: mxc ipuv3 fb: Use fb_set_var() to unblank a framebuffer

The resolution of an overlay framebuffer could be at most the same to the
one of the relevant background framebuffer.  However, the resolution of a
HDMI monitor could be changed at runtime.  Thus, when we unblank the overlay
framebuffer, we should check it's resolution and start position in case it
exceeds the boundary of the background framebuffer.  This patch replaces
mxcfb_set_par() with fb_set_var() to implement the unblank operation so that
the logic contains the ->fb_check_var() path.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>