drm/amdgpu: Update function level documentation for GPUVM v3

Add/update function level documentation and add reference to amdgpu_vm.c
in amdgpu.rst

v2:
Fix reference in rst file.
Fix compilation warnings.
Add space between function names and params list where
it's missing.

v3:
Fix some funtion comments.
Add formatted documentation to structs.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>