drm/radeon: fix checking of MSAA renderbuffers on r600-r700

The MSAA checking was mostly unimplemented on r600-r700. The userspace
submits GPU commands and the kernel driver computes how much memory
the GPU will access and checks if it's all within buffer bounds the
userspace allocated. This patch fixes the computations of the size of
MSAA surfaces in memory.

Signed-off-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>