vc_screen: extract vcs_read_buf_uni

The same as making write more readable, extract unicode handling from
vcs_read. The other two cases (w/ and w/o attributes) will follow.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200818085706.12163-12-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>