Add more generic function amdgpu_copy_ttm_mem_to_mem() that supports
arbitrary copy size, offsets and two BOs (source & dest.).

This is useful for KFD Cross Memory Attach feature where data needs to
be copied from BOs from different processes

v2: Add struct amdgpu_copy_mem and changed amdgpu_copy_ttm_mem_to_mem()
function parameters to use the struct

v3: Minor function name change
Signed-off-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>