drm/i915: Constify all plane_funcs structs

plane_funcs can be cosnt. Make them so.

v2: Rebase due to per-platforms plane_funcs
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005125817.22576-2-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/intel_display.c

@@ -13530,7 +13530,7 @@ static bool intel_cursor_format_mod_supported(struct drm_plane *_plane,
 		format == DRM_FORMAT_ARGB8888;
 }
 
-static struct drm_plane_funcs skl_plane_funcs = {
+static const struct drm_plane_funcs skl_plane_funcs = {
 	.update_plane = drm_atomic_helper_update_plane,
 	.disable_plane = drm_atomic_helper_disable_plane,
 	.destroy = intel_plane_destroy,
@@ -13541,7 +13541,7 @@ static struct drm_plane_funcs skl_plane_funcs = {
 	.format_mod_supported = skl_plane_format_mod_supported,
 };
 
-static struct drm_plane_funcs i965_plane_funcs = {
+static const struct drm_plane_funcs i965_plane_funcs = {
 	.update_plane = drm_atomic_helper_update_plane,
 	.disable_plane = drm_atomic_helper_disable_plane,
 	.destroy = intel_plane_destroy,
@@ -13552,7 +13552,7 @@ static struct drm_plane_funcs i965_plane_funcs = {
 	.format_mod_supported = i965_plane_format_mod_supported,
 };
 
-static struct drm_plane_funcs i8xx_plane_funcs = {
+static const struct drm_plane_funcs i8xx_plane_funcs = {
 	.update_plane = drm_atomic_helper_update_plane,
 	.disable_plane = drm_atomic_helper_disable_plane,
 	.destroy = intel_plane_destroy,