vc_screen: extract vcs_write_buf

vc_screen: extract vcs_write_buf

This is the counterpart of the previous patch: here, we extract buffer
writing with attributes from vcs_write.

Now, there is no need for org to be initialized to NULL. The org0
check before update_region() confuses compilers, so check org instead.
It provides the same semantics. And it also eliminates the need for
initialization of org0.

We switch the branches of the attr 'if' too, as the inversion brings only
confusion now.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200818085706.12163-8-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>