regulator: core: Check for DT every time we check full constraints

Eliminate the gap between DT becoming available and this being used to say
we have full constraints by checking directly for DT every time we check
for full constraints. This improves interoperaton with optional regulator
support.

Signed-off-by: Mark Brown <broonie@linaro.org>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>