drm/i915: Use adjusted_mode in HDMI 12bpc clock check

The pixel clock should come from adjusted_mode not requested_mode.
In this case the two should be the same as we don't currently
overwrite the clock in the case of HDMI. But let's make the code
safe against such things happening in the future.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>