drm/arm: mark symbols static where possible

drm/arm: mark symbols static where possible

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/arm/malidp_planes.c:49:25: warning: no previous prototype for 'malidp_duplicate_plane_state' [-Wmissing-prototypes]
drivers/gpu/drm/arm/malidp_planes.c:66:6: warning: no previous prototype for 'malidp_destroy_plane_state' [-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 these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>