vc_screen: extract vcs_write_buf_noattr

vcs_write is too long to be readable. Extract buffer handling w/o
attributes from there to a separate function.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200818085706.12163-7-jslaby@suse.cz


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