drm/amdgpu: Fix amdgpu_pm_acpi_event_handler warning

Include a missing header to get rid of the following warning:

drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c:65:6: warning: no previous prototype for ‘amdgpu_pm_acpi_event_handler’ [-Wmissing-prototypes]
 void amdgpu_pm_acpi_event_handler(struct amdgpu_device *adev)
      ^

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c

@@ -30,10 +30,10 @@
 #include <drm/drmP.h>
 #include <drm/drm_crtc_helper.h>
 #include "amdgpu.h"
+#include "amdgpu_pm.h"
 #include "amd_acpi.h"
 #include "atom.h"
 
-extern void amdgpu_pm_acpi_event_handler(struct amdgpu_device *adev);
 /* Call the ATIF method
  */
 /**

drivers/gpu/drm/amd/amdgpu/amdgpu_pm.h

@@ -30,6 +30,7 @@ struct cg_flag_name
 	const char *name;
 };
 
+void amdgpu_pm_acpi_event_handler(struct amdgpu_device *adev);
 int amdgpu_pm_sysfs_init(struct amdgpu_device *adev);
 void amdgpu_pm_sysfs_fini(struct amdgpu_device *adev);
 void amdgpu_pm_print_power_states(struct amdgpu_device *adev);