drm/radeon: Disable HDP flush before every CS again for < r600

It was causing display corruption with R300 generation GPUs at least.

Reported-and-Tested-by: Mikael Pettersson <mikpelinux@gmail.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>