drm/i915/dsi: use afe_clk() instead of intel_dsi_bitrate()

We'll be expanding afe_clk() to take DSC into account. Switch to using
it where DSC matters. Which is really everywhere that
intel_dsi_bitrate() is currently used in ICL DSI code.

The functional difference is that we round the result closest instead of
down.

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b6c52b320daa8aaa0d79618ce714170f8f04ff67.1575974743.git.jani.nikula@intel.com