drm/i915/bxt: Wait for PHY1 GRC calibration synchronously

A follow-up patch moves the PHY enabling to the power well code where
enabling/disabling the PHYs will happen independently. Because of this
waiting for the GRC calibration in PHY1 asynchronously would need some
additional logic. Instead of adding that let's keep things simple for now
and wait synchronously. My measurements showed that the calibration
takes ~4ms.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>