drm/docs: Fix documentation for drm_atomic_set_mode_for_crtc

drm_atomic_set_mode_for_crtc() modifies the .enable member of CRTC state
but documentation claims otherwise, fix that.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1485843518-28438-1-git-send-email-dhinakaran.pandiyan@intel.com