drm: xilinx: dsi: Add const for constant callback functions

drm: xilinx: dsi: Add const for constant callback functions

From checkpatch.pl,

WARNING: struct drm_connector_funcs should normally be const
+static struct drm_connector_funcs xilinx_dsi_connector_funcs = {
WARNING: struct drm_encoder_helper_funcs should normally be const
+static struct drm_encoder_helper_funcs xilinx_dsi_encoder_helper_funcs = {
WARNING: struct drm_encoder_funcs should normally be const
+static struct drm_encoder_funcs xilinx_dsi_encoder_funcs = {

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>