Move struct amdgpu_wb and helpers out of the monolithic header amdgpu.h
into its own dedicated header amdgpu_wb.h.

Add amdgpu_wb_init() and amdgpu_wb_fini() into amdgpu_wb.h.

Move functions amdgpu_device_wb_get(), amdgpu_device_wb_free(),
amdgpu_device_wb_init(), and amdgpu_device_wb_fini() out of
amdgpu_device.c into new dedicated amdgpu_wb.c file.

Removed static from functions amdgpu_device_wb_init() and amdgpu_device_wb_fini().

Rename functions amdgpu_device_wb_get(), amdgpu_device_wb_free(),
amdgpu_device_wb_init(), and amdgpu_device_wb_fini() into
amdgpu_wb_get(), amdgpu_wb_free(), amdgpu_wb_init(), and amdgpu_wb_fini().

Update amdgpu/Makefile to build amdgpu_wb.o.

This is part of the ongoing effort to reduce the size of amdgpu.h into
their own respective separate headers.

Signed-off-by: Shahyan Soltani <shahyan.soltani@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>