drm/i915: Round the AUX clock divider to closest on all platforms

drm/i915: Round the AUX clock divider to closest on all platforms

Currently we round the AUX clock divider down on g4x, to closest
on HSW/BDW port A, and up everywhere else. We are supposed to get
as close to 2MHz as we can, so round to closest seems like the
best option.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448893432-6978-6-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>