drm/i915/bxt: edp1.4 Intermediate Freq support

BXT supports following intermediate link rates for edp:
2.16GHz, 2.43GHz, 3.24GHz, 4.32GHz.
Adding support for programming the intermediate rates.

v2: Adding clock in bxt_clk_div struct and then look for the entry with
required rate (Ville)
v3: 'clock' has the selected value, no need to use link_bw or rate_select
for selecting pll(Ville)
v4: Make bxt_dp_clk_val const and remove size (Ville)
v5: Rebased
v6: Removed setting of vco while rebasing in v5, adding it back

Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v4)
Reviewed-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>