drm/amdgpu/ttm: Use GART helper to map VRAM pages (v2)

drm/amdgpu/ttm: Use GART helper to map VRAM pages (v2)

Use the GART helper function introduced in the previous commit
to map the VRAM pages of the transfer window to GART.
No functional changes, just code cleanup.

Split this into a separate commit to make it easier to bisect,
in case there are problems in the future.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>