drm/amdgpu: cleanup amdgpu_ttm_copy_mem_to_mem and amdgpu_map_buffer v2

Cleanup amdgpu_ttm_copy_mem_to_mem by using fewer variables
for the same value.

Rename amdgpu_map_buffer to amdgpu_ttm_map_buffer, move it
to avoid the forward decleration, cleanup by moving the map
decission into the function and add some documentation.

No functional change.

v2: add some more cleanup suggested by Felix
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Tested-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.h

@@ -89,8 +89,8 @@ int amdgpu_copy_buffer(struct amdgpu_ring *ring, uint64_t src_offset,
 		       struct dma_fence **fence, bool direct_submit,
 		       bool vm_needs_flush, bool tmz);
 int amdgpu_ttm_copy_mem_to_mem(struct amdgpu_device *adev,
-			       struct amdgpu_copy_mem *src,
-			       struct amdgpu_copy_mem *dst,
+			       const struct amdgpu_copy_mem *src,
+			       const struct amdgpu_copy_mem *dst,
 			       uint64_t size, bool tmz,
 			       struct dma_resv *resv,
 			       struct dma_fence **f);