[hub75] Fix gamma_correct to use enum value instead of key string (#13102)

Co-authored-by: Claude <noreply@anthropic.com>

esphome/components/hub75/display.py

@@ -555,7 +555,7 @@ async def to_code(config: ConfigType) -> None:
         cg.add_build_flag(f"-DHUB75_BIT_DEPTH={config[CONF_BIT_DEPTH]}")
 
     if CONF_GAMMA_CORRECT in config:
-        cg.add_build_flag(f"-DHUB75_GAMMA_MODE={config[CONF_GAMMA_CORRECT]}")
+        cg.add_build_flag(f"-DHUB75_GAMMA_MODE={config[CONF_GAMMA_CORRECT].enum_value}")
 
     # Await all pin expressions
     pin_expressions = {

tests/components/hub75/test.esp32-s3-idf-board.yaml

@@ -6,6 +6,7 @@ display:
     panel_height: 32
     double_buffer: true
     brightness: 128
+    gamma_correct: gamma_2_2
     pages:
       - id: page1
         lambda: |-

tests/components/hub75/test.esp32-s3-idf.yaml

@@ -5,6 +5,7 @@ display:
     panel_height: 32
     double_buffer: true
     brightness: 128
+    gamma_correct: cie1931
     r1_pin: GPIO42
     g1_pin: GPIO41
     b1_pin: GPIO40