drm/i915: Warn when cdclk for the platforms is not known

Print a warning if we fall through the .get_display_clock_speed() function
pointer setup. We end up assuming a 133MHz cdclk which should mean that
at least we avoid any 0 deivisions and whatnot. But this could at least
help remind people that they have to provide this function for new platforms.

v2: Rebased to the latest
v3: Rebased to the latest

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v1)
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>