drm/i915: Sync mode_valid/mode_set with intel video driver

This covers:
Limit CRT DAC speed better.

and also clears the border color in case it's set to some garbage, which would
fix ugly outlines in the blank regions of the CRT.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
[anholt: replaced *drm_dev with *dev]
Signed-off-by: Eric Anholt <eric@anholt.net>