staging: fbtft: Use kmemdup rather than duplicating its implementation

The semantic patch that makes this change is available
in scriptcoccinelle/api/memdup.cocci.

Signed-off-by: Madhusudhanan Ravindran <mravindran04@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>