staging: sm7xx: rename function

This patch changes the name of function cfb_blank to smtc_blank. This
function is not implemented like a generic function for frame buffer.
Having the old prefix becomes confused when reading fb_ops structs.

Tested with SM712

Signed-off-by: Javier M. Mellid <jmunhoz@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>