drm: modes: add missing [drm] to message printing

The warning message in drm_mode_parse_command_line_for_connector miss
the [drm] at beginning.
This patch add it and take the opportunity to convert
printk(KERN_WARNING to pr_warn()

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1454594633-15100-1-git-send-email-clabbe.montjoie@gmail.com