drm/i915/dp: deconflate PPS unlock from divisor register

PPS locking is a thing on pre-DDI, up to and including CPT and PPT.

The PPS divisor register exists up to gen 9 BC, replaced by a field in
the control register starting from gen 9 LP, i.e. BXT, GLK, and CNP on.

Commit b0a08bec ("drm/i915/bxt: eDP Panel Power sequencing") stopped
using the divisor register, but inadvertently conflated the PPS unlock
in the change. No longer doing the unlocking was the right thing to do,
however we should've stopped already at LPT (or DDI platforms).

Deconflate the two.

Arguably this could be moved away from here altogether, but this is the
minimally intrusive change for now.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190305135215.29862-1-jani.nikula@intel.com