drm/i915/dsi: double check element parsing against size if present

Be a little paranoid in case the specs change or something.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/27050d48d0ff3d58e79fcacc41ced4c071b01424.1474286487.git.jani.nikula@intel.com

drivers/gpu/drm/i915/intel_dsi_panel_vbt.c

@@ -441,7 +441,15 @@ static void generic_exec_sequence(struct drm_panel *panel, enum mipi_seq seq_id)
 			operation_size = *data++;
 
 		if (mipi_elem_exec) {
+			const u8 *next = data + operation_size;
+
 			data = mipi_elem_exec(intel_dsi, data);
+
+			/* Consistency check if we have size. */
+			if (operation_size && data != next) {
+				DRM_ERROR("Inconsistent operation size\n");
+				return;
+			}
 		} else if (operation_size) {
 			/* We have size, skip. */
 			DRM_DEBUG_KMS("Unsupported MIPI operation byte %u\n",