drm/i915/bxt: mask off the DPLL state checker bits we don't program

For the purpose of state checking we only care about the DPLL HW flags
that we actually program, so mask off the ones that we don't.

This fixes one set of DPLL state check failures.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>