drm/i915: Mask out hardware status bits from VLV DPLL register

The DPLL lock bit, and the DPIO phy status bits are read-only and
controlled by the hardware, so they will never be set by the driver.
Mask them out when reading the hw state, so that the state
comparison won't fail.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuosugeek.org>
[danvet: Jesse asked for a code comment and I wholeheartly agree, so
added one.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>