drm/amdgpu: remove unnecessary forward declaration

struct vi_sdma_mqd is defined in vi_structs.h.
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gfx_v8.c

@@ -45,8 +45,6 @@ enum hqd_dequeue_request_type {
 	RESET_WAVES
 };
 
-struct vi_sdma_mqd;
-
 /*
  * Register access functions
  */