drm/amdkfd: pages_addr offset must be 0 for system range

prange->offset is for VRAM range mm_nodes, if multiple ranges share same
mm_nodes, migrate range back to VRAM will reuse the VRAM at offset of
the same mm_nodes. For system memory pages_addr array, the offset is
always 0, otherwise, update GPU mapping will use incorrect system memory
page, and cause system memory corruption.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>