drm/radeon: mark symbols static where possible

We get 4 warnings when building kernel with W=1:
drivers/gpu/drm/radeon/si.c:7850:5: warning: no previous prototype for 'si_vce_send_vcepll_ctlreq' [-Wmissing-prototypes]
drivers/gpu/drm/radeon/radeon_dp_mst.c:226:21: warning: no previous prototype for 'radeon_mst_best_encoder' [-Wmissing-prototypes]
drivers/gpu/drm/radeon/radeon_dp_mst.c:344:26: warning: no previous prototype for 'radeon_mst_find_connector' [-Wmissing-prototypes]
drivers/gpu/drm/radeon/radeon_dp_mst.c:600:6: warning: no previous prototype for 'radeon_dp_mst_encoder_destroy' [-Wmissing-prototypes]

In fact, these 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'.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>