drm/ast: Fix test for VGA enabled

The test to see if VGA was already enabled is doing an unnecessary
second test from a register that may or may not have been initialized
to a valid value. Remove it.

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: <stable@vger.kernel.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>