drm: bridge: analogix/dp: mark symbols static where possible

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/bridge/analogix/analogix_dp_core.c:1053:5: warning: no previous prototype for 'analogix_dp_get_modes' [-Wmissing-prototypes]
drivers/gpu/drm/bridge/analogix/analogix_dp_core.c:1097:1: warning: no previous prototype for 'analogix_dp_detect' [-Wmissing-prototypes]

In fact, both functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
So this patch marks both functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1474788764-6069-1-git-send-email-baoyou.xie@linaro.org