msm/mdp5: Fix some kernel-doc warnings

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/msm/disp/mdp5/mdp5_ctl.c:227: warning: Function parameter or member 'ctl' not described in 'mdp5_ctl_set_encoder_state'
 drivers/gpu/drm/msm/disp/mdp5/mdp5_ctl.c:227: warning: Function parameter or member 'pipeline' not described in 'mdp5_ctl_set_encoder_state'
 drivers/gpu/drm/msm/disp/mdp5/mdp5_ctl.c:227: warning: Function parameter or member 'enabled' not described in 'mdp5_ctl_set_encoder_state'
 drivers/gpu/drm/msm/disp/mdp5/mdp5_ctl.c:227: warning: Excess function parameter 'enable' description in 'mdp5_ctl_set_encoder_state'
 drivers/gpu/drm/msm/disp/mdp5/mdp5_ctl.c:529: warning: Function parameter or member 'ctl' not described in 'mdp5_ctl_commit'
 drivers/gpu/drm/msm/disp/mdp5/mdp5_ctl.c:529: warning: Function parameter or member 'pipeline' not described in 'mdp5_ctl_commit'
 drivers/gpu/drm/msm/disp/mdp5/mdp5_ctl.c:529: warning: Function parameter or member 'flush_mask' not described in 'mdp5_ctl_commit'
 drivers/gpu/drm/msm/disp/mdp5/mdp5_ctl.c:529: warning: Function parameter or member 'start' not described in 'mdp5_ctl_commit'

Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>