drm/radeon: fix UVD 256MB check

commit bae651dbd7ade3c5d6518f89599ae680a2fe2b85 upstream.

Otherwise the kernel might reject our decoding requests.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>