vc_screen: extract vcs_read_buf

And finally, move the attributes buffer handling to a separate function.
Leaving vcs_read quite compact.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200818085706.12163-14-jslaby@suse.cz


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