drm/i915: Make downclock deduction common for all panels

drm/i915: Make downclock deduction common for all panels

If one mode of a internal panel has more than one refresh rate, then a reduced
clock is found for the LFP (LVDS/eDP). This enables switching between low
and high frequency dynamically. Moving downclock calculation to intel_panel
so that it is common for LVDS and eDP.

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Pradeep Bhat <pradeep.bhat@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>