drm/i915: Unify the DPLL ref clock frequency tracking

drm/i915: Unify the DPLL ref clock frequency tracking

All platforms using the shared DPLL framework use 3 reference clocks for
their DPLLs: SSC, non-SSC and DSI. For a more unified way across
platforms store the frequency of these ref clocks as part of the DPLL
global state. This also allows us to keep the HW access reading out the
ref clock value separate from the DPLL frequency calculation that
depends on the ref clock.

For now add only the SSC and non-SSC ref clocks, as the pre-ICL DSI code
has its own logic for calculating DPLL parameters instead of the shared
DPLL framework.

v2:
- Apply the ICL combo PHY PLL ref_clock/2 adjustment during the
  frequency->PLL param conversion direction as well. (CI shards)
- s/kHZ/kHz/ (Ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228153328.17842-1-imre.deak@intel.com