drm/ttm: Fix 'buf' pointer update in ttm_bo_vm_access_kmap() (v2)

The buf pointer was not being incremented inside the loop
meaning the same block of data would be read or written
repeatedly.

(v2) Change 'buf' pointer to uint8_t* type

Cc: stable@vger.kernel.org
Fixes: 09ac4fcb ("drm/ttm: Implement vm_operations_struct.access v2")

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>