drm/i915: Add state readout and checking for has_dp_encoder and dp_m_n

Add functions to read out the CPU and PCH transcoder M/N values,
and use them to fill out the pipe config dp_m_n information. And
while at it populate has_dp_encoder too.

Also refactor ironlake_get_fdi_m_n_config() to simply call the new
intel_cpu_transcoder_get_m_n() function.

v2: Remember the DDI

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>