In suspend/resume and HDMI plugin/plugout stress test,
sometimes fbcon will call fb_set_par with
parameter fb_var_screeninfo that xres anfd yres is zero.
MX frame buffer driver can not correct handle this casue,
it will cause IPU pixel clock gating/ungating mismatch.

Check fb_var_screeninfo parameter in mxcfb_check_var and
mxcfb_set_par function, returned if xres,yres zero.

Signed-off-by: Sandor Yu <R01008@freescale.com>