drm/i915/dsi: abstract get pclk platform differences

Hide away the platform differences in intel_dsi_get_pckl() within
intel_dsi_pll.c. No functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452249940-2605-1-git-send-email-jani.nikula@intel.com