drm/i915/chv: Fix CHV PLL state tracking

Setup the pipe config dpll state correctly for CHV. Also add
a assert_pipe_disabled() to chv_disable_pll(), and program the
DPLL_MD registers in chv_enable_pll().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>