drm/amd/powerplay: add missing header file declaration

This can fix the compile errors below:
drivers/gpu/drm/amd/amdgpu/../powerplay/smu_v11_0.c: In function ‘smu_v11_0_baco_set_state’:
drivers/gpu/drm/amd/amdgpu/../powerplay/smu_v11_0.c:1674:27: error: implicit declaration of function ‘amdgpu_ras_get_context’ [-Werror=implicit-function-declaration]
  struct amdgpu_ras *ras = amdgpu_ras_get_context(adev);
                           ^
drivers/gpu/drm/amd/amdgpu/../powerplay/smu_v11_0.c:1674:27: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
drivers/gpu/drm/amd/amdgpu/../powerplay/smu_v11_0.c:1692:19: error: dereferencing pointer to incomplete type ‘struct amdgpu_ras’
   if (!ras || !ras->supported) {

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>