drm/i915/psr: Make intel_psr_enable_sink as local static function

Intel_psr_enable_sink is not used outside intel_psr.c. Convert it as local
static function.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250109103532.2093356-4-jouni.hogander@intel.com