drm/i915/icl: Simplify release of encoder power refs

We can unconditionally release the power references during encoder
disabling. The references for each port used by the encoder are
guaranteed to be enabled at this point.

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405153657.20921-2-imre.deak@intel.com