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

drivers/gpu/drm/i915/intel_ddi.c

@@ -1021,7 +1021,7 @@ static u32 icl_pll_to_ddi_pll_sel(struct intel_encoder *encoder,
 			return DDI_CLK_SEL_TBT_810;
 		default:
 			MISSING_CASE(clock);
-			break;
+			return DDI_CLK_SEL_NONE;
 		}
 	case DPLL_ID_ICL_MGPLL1:
 	case DPLL_ID_ICL_MGPLL2: