drm/edid: Round to closest when computing the CEA/HDMI alternate clock

Rounding to the closest kHz seems like the better option that round
down or up when computing the alternate clock for CEA/HDMI modes.
It'll give us a slightly more accurate clock in some cases.

Not sure why I went for the down+up approach originally. Perhaps
I was thinking we can go back and forth betwen the two frequencies
without introducing errors, but round to closest still maintains
that property.

Cc: Adam Jackson <ajax@redhat.com>
Cc: Clint Taylor <clinton.a.taylor@intel.com>
Cc: Libin Yang <libin.yang@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>