ENGR00210915 fb: mxc_elcdfb: fix screen flash when use fb_set_var

ENGR00210915 fb: mxc_elcdfb: fix screen flash when use fb_set_var

fb_set_var() is used by android default display,
we found the screen will flash using android default
display support, found the root case was we init the
hardware (blank and unblank) the screen.

because fb_set_var() will call fb_set_par() every time,
we needs check only set the hardware register when needed,
then the android default image without any modify can works.

Also a build warnning fix.

Signed-off-by: Zhang Jiejing <jiejing.zhang@freescale.com>
Signed-off-by: Liu Ying <Ying.Liu@freescale.com>