drm/i915: clock readout support for DDI v3

Read out and calculate the port and pixel clocks on DDI configs as well.
This means we have to grab the DP divider values and look at the port
mapping to figure out which clock select reg to read out.

v2: do the work from ddi_get_config (Ville)
v3: check WRPLL reference clock (Ville)
    add additional SPLL freqs (Ville)
    clean up port/crtc clock calc (Ville)
    fix up crtc_clock conditionals (Ville)
    drop superfluous dp_get_m_n from get_config (Ville)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>