drm/i915: Pick the first unused PLL once again

commit 5b0bd14d ("drm/i915/icl: keep track of unused pll while
looping") inadvertently (I presume) changed the code to pick the
last unused dpll rather than the first unused one like we did before.

While there should most likely be no harm in changing the order
let's change back just to avoid a change in the behaviour. At
least it might reduce the confusion when staring at logs (took
me a while to figure out why DPLL1 being picked over DPLL0
when the latter was most definitely available).

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190130181359.20693-1-ville.syrjala@linux.intel.comReviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Acked-by: NLucas De Marchi <lucas.demarchi@intel.com>

drivers/gpu/drm/i915/intel_dpll_mgr.c

@@ -258,7 +258,8 @@ intel_find_shared_dpll(struct intel_crtc *crtc,
 
 		/* Only want to check enabled timings first */
 		if (shared_dpll[i].crtc_mask == 0) {
-			unused_pll = pll;
+			if (!unused_pll)
+				unused_pll = pll;
 			continue;
 		}