drm/msm/disp/mdp5/mdp5_kms: Make local functions 'mdp5_{en, dis}able()' static

drm/msm/disp/mdp5/mdp5_kms: Make local functions 'mdp5_{en, dis}able()' static

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

 drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c:299:5: warning: no previous prototype for ‘mdp5_disable’ [-Wmissing-prototypes]
 drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c:319:5: warning: no previous prototype for ‘mdp5_enable’ [-Wmissing-prototypes]

Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: linux-arm-msm@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: freedreno@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>