drm/ast: Factor mmc_test code in POST code

There's a some duplication for what's essentially copies of
two loops, so factor it. The upcoming AST2500 POST code adds
more of them. Also cleanup return types for the test functions,
most of them return a boolean, some return a u32.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Joel Stanley <joel@jms.id.au>
Tested-by: Y.C. Chen <yc_chen@aspeedtech.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>