drm/amdkfd: Move packet writer functions into ASIC-specific file

This is in preparation for GFXv9 (Vega10) which uses incompatible PM4
packet formats from previous ASIC generations.

Signed-off-by: Shaoyun Liu <Shaoyun.Liu@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>