drm/amd/dc: Use forward declaration instand of include header file

drm/amd/dc: Use forward declaration instand of include header file

avoid build error:

drivers/gpu/drm/amd/amdgpu/../powerplay/inc/smu9_driver_if.h:342:3: error: redeclaration of enumerator ‘WM_COUNT’
   WM_COUNT,
   ^
In file included from drivers/gpu/drm/amd/amdgpu/../display/dc/dm_services_types.h:32:0,
                 from drivers/gpu/drm/amd/amdgpu/../display/dc/dm_services.h:35,
                 from drivers/gpu/drm/amd/amdgpu/../display/modules/inc/mod_freesync.h:57,
                 from drivers/gpu/drm/amd/amdgpu/../amdgpu/amdgpu_mode.h:48,
                 from drivers/gpu/drm/amd/amdgpu/../amdgpu/amdgpu.h:55,
                 from drivers/gpu/drm/amd/amdgpu/../powerplay/inc/amd_powerplay.h:33,
                 from drivers/gpu/drm/amd/amdgpu/../powerplay/inc/smumgr.h:26,
                 from drivers/gpu/drm/amd/amdgpu/../powerplay/smumgr/vega10_smumgr.c:24:
drivers/gpu/drm/amd/amdgpu/../display/dc/dm_pp_smu.h:43:2: note: previous definition of ‘WM_COUNT’ was here
  WM_COUNT,

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>