drm/amdgpu: Clear VCN cache when hw_init

[Why]
VCN will use some framebuffer space as its cache. It needs to
be reset when reset happens, such as FLR. Otherwise some error
may be kept after the reset.

Signed-off-by: Horace Chen <horace.chen@amd.com>
Reviewed-by: Emily Deng <Emily.Deng@amd.com>