drm/i915: Add CD and pixel clock information

This patch adds information of current and maximum CD clock
frequency and pixel clock frequency information on 'i915_debugfs.c'.

v2:
- combined seperate patches for current CD clock, maximum CD clock
  and maximum pixel clock
- space added between the frequency value and the unit

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>