video: mxsfb: convert pr_debug()/dev_dbg() to pr_err()/dev_err() for error messages

Make the messages that are printed in case of fatal errors actually
visible to the user without having to recompile the driver with
debugging enabled.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>