drm/i915/icl: keep track of unused pll while looping

Instead of looping again on the range of plls, just keep track of one
unused one and use it later.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125222444.19926-5-lucas.demarchi@intel.com