drm/i915: Drop checks for max_pixclk failures in cdclk computation

commit 565602d7

 ("drm/i915: Do not acquire crtc state to check clock during modeset, v4.")
removed the possibility that intel_mode_max_pixclk() or
ilk_max_pixel_rate() might return an error, so let's get rid of the
error checks in the callers as well.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462995892-32416-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>