fixes attribute error for AXS15231B display driver.

Author: kdschlosser

api_drivers/common_api_drivers/display/axs15231b/axs15231b.py

@@ -52,7 +52,7 @@ def __init__(
     ):
         self.__tx_color_count = 0
         self._brightness = 0xD0
-        color_size = lv.color_format_get_size(self._color_space)
+        color_size = lv.color_format_get_size(color_space)
         buf_size = display_width * display_height * color_size
         if isinstance(data_bus, lcd_bus.RGBBus):
             buf_size = int(buf_size // 10)