drm/amdgpu: Add empty HDP flush function to JPEG v4.0.3

JPEG v4.0.3 doesn't support HDP flush when RRMT is enabled. Instead,
mmsch fw will do the flush.

This change is necessary for JPEG v4.0.3, no need for backward compatibility

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Jane Jian <Jane.Jian@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>