drm: Introduce a crtc_clock for struct drm_display_mode

Just like the various timings, make it possible to have a clock field
what we can tweak before giving it to hardware.

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