drm/amdgpu/ttm: replace duplicate code with exiting function

using exiting function to replace duplicate code blocks in
amdgpu_ttm_vram_write().

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>