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>
diff --git a/drivers/gpu/drm/radeon/atombios_encoders.c b/drivers/gpu/drm/radeon/atombios_encoders.c
index f3645e1..370b2c4 100644
--- a/drivers/gpu/drm/radeon/atombios_encoders.c
+++ b/drivers/gpu/drm/radeon/atombios_encoders.c
@@ -72,7 +72,7 @@
 		WREG32(RADEON_BIOS_2_SCRATCH, bios_2_scratch);
 }
 
-static void
+void
 atombios_set_panel_brightness(struct radeon_encoder *radeon_encoder)
 {
 	struct drm_encoder *encoder = &radeon_encoder->base;