drm/amdkfd: Use Arcturus specific set_vm_context_page_table_base()

Since Arcturus has it own function pointer, we can move Arcturus
specific logic to there rather than leaving it entangled with
other GFX9 chips.

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