drm/i915/dp: remove PANEL_POWER_OFF macro and its use

It's superfluous.

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7987938a7950853ac3ee43c82fb9cbb0cd59a2fa.1547726792.git.jani.nikula@intel.com