drm/msm: remove the drm_device_is_unplugged check

This flag is only used for drm/udl.

Signed-off-by: Haixia Shi <hshi@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1454709464-2536-1-git-send-email-hshi@chromium.org