fbdev: sh_mobile_lcdcfb: add blanking support

Add a blanking callback to the LCDC driver in order to support both
FBIOBLANK and TIOCLINUX blanking ioctls. LCDC clocks are also released
if the requested blanking level is superior to FB_BLANK_NORMAL, to allow
runtime PM to disable the clocks if possible.

Signed-off-by: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>