drm/i915: always return something on DDI clock selection

Even if we don't have the correct clock and get a warning, we should not
skip the return.

v2: improve commit message (from Joonas)

Fixes: 1fa11ee2 ("drm/i915/icl: start adding the TBT pll")
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: <stable@vger.kernel.org> # v4.19+
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125222444.19926-3-lucas.demarchi@intel.com


(cherry picked from commit 7a61a6de)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>