drm/amdgpu: Fix the kerneldoc description

amdgpu_ttm_tt_set_userptr() is also called by the KFD as part of
initializing the user pages for userptr BOs and also while initializing
the GPUVM for a KFD process so update the function description.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>