drm/i915: Print a debug message when exceeding dotclock limit on pre-gen4

Currently there's no trace in dmesg when the gen2/3 dotclock checks
reject the modeset. Add some to avoid further head scratching.

While at it refactor the code a bit to look nicer.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446241178-432-1-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>