gpu: drm: Fix warning caused by a parameter description in drm_crtc.c

This patch fix following error while "make xmldocs"

Warning(.//drivers/gpu/drm/drm_crtc.c:778): Excess function parameter
 'mode' description in 'drm_connector_get_cmdline_mode'

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>