drm/log: Do not draw if drm_master is taken

When userspace takes drm_master, the drm_client buffer is no more
visible, so drm_log shouldn't waste CPU cycle to draw on it.

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20241204160014.1171469-4-jfalempe@redhat.com