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>