drm/radeon: implement handler for ACPI event

Set up an handler for ACPI events and respond to brightness change
requests from the system BIOS.
v2: fix notification when using device-specific command codes
(tested by Pali Rohár <pali.rohar@gmail.com>); cache the encoder
controlling the backlight during the initialization to avoid searching
it every time (suggested by Alex Deucher).
v3: whitespace fixes (Alex Deucher).

Signed-off-by: Luca Tettamanti <kronos.it@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_pm.c

@@ -22,6 +22,7 @@
  */
 #include "drmP.h"
 #include "radeon.h"
+#include "radeon_acpi.h"
 #include "avivod.h"
 #include "atom.h"
 #ifdef CONFIG_ACPI
@@ -94,7 +95,8 @@ static int radeon_acpi_event(struct notifier_block *nb,
 		}
 	}
 
-	return NOTIFY_OK;
+	/* Check for pending SBIOS requests */
+	return radeon_atif_handler(rdev, entry);
 }
 #endif