The function cnl_ddi_dp_set_dpll_hw_state does not need to be in global
scope, so make it static.

Cleans up sparse warning:
"symbol 'cnl_ddi_dp_set_dpll_hw_state' was not declared. Should it
 be static?"
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170613134751.29196-1-colin.king@canonical.comReviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>