drm/radeon/kms: add MSAA texture support for r600-evergreen

Most of the checking seems to be in place already. As you can see,
log2(number of samples) resides in LAST_LEVEL.

This is required for MSAA support (namely for depth-stencil resolve and
blitting between MSAA resources).

Signed-off-by: Marek Olšák <maraeo@gmail.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>