fbdev: imxfb: use __func__ for function name

Resolve the following warning reported by checkpatch:

WARNING: Prefer using '"%s...", __func__' to using 'imxfb_blank', this function's name, in a string

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Helge Deller <deller@gmx.de>