drm/pl111/pl111_display: Make local function static

drm/pl111/pl111_display: Make local function static

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

 drivers/gpu/drm/pl111/pl111_display.c:356:6: warning: no previous prototype for ‘pl111_display_disable’ [-Wmissing-prototypes]

Cc: Eric Anholt <eric@anholt.net>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-23-lee.jones@linaro.org