vt: remove superfluous parens in invert_screen and build_attr

There were too many parentheses in invert_screen, remove them and align
the code in invert_screen a bit.

No functional change intended.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200615074910.19267-27-jslaby@suse.cz


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