drm/radeon: Only flush HDP cache from idle ioctl if BO is in VRAM

The HDP cache only applies to CPU access to VRAM.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>