drm/amdgpu: use the BAR if possible in amdgpu_device_vram_access v2

This should speed up debugging VRAM access a lot.

v2: add HDP flush/invalidate

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Jonathan Kim <Jonathan.Kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>