video: mxsfb: Fix reset hang when videomode variable is not present

video: mxsfb: Fix reset hang when videomode variable is not present

Currently the system hangs when the 'videomode' variable is not present
and a reset command is issued:

=> setenv videomode
=> saveenv
=> reset

(Board hangs)

lcdif_power_down() assumes that the LCDIF controller has been properly
configured and enabled, which may not be true.

To fix this issue check whether panel.frameAdrs has been initialized and
in case it has not been initialized, do not continue with the LCDIF
powerdown sequence.

Tested on a imx7dsabresd board.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Anatolij Gustschin <agust@denx.de>