drm/i915: Store power sequencer delays in intel_dp

The power seqeuncer delays are fixed for a given panel, so we can keep
them around once computed.

Not that on VLV/CHV we still re-compute them every time we initialize
the power seqeuncer registers, but that will change soon enough.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>