drivers/gpu/drm/qxl/Kconfig: reformat the help text

Reformat the help text to follow the style of the other Kconfig files,
and also fix indentation for the "select" lines.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>