drm/i915: correct intel_dp_get_config() function for DevCPT

On DevCPT, the control register for Transcoder DP Sync Polarity is
TRANS_DP_CTL, not DP_CTL.
Without this patch, Many call trace occur on CPT machine with DP monitor.
The call trace is like: *ERROR* mismatch in adjusted_mode.flags(expected X,found X)

v2: use intel-crtc to simple patch, suggested by Daniel.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
[danvet: Extend the encoder->get_config comment to specify that we now
also depend upon intel_encoder->base.crtc being correct. Also bikeshed
s/intel_crtc/crtc/.]
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65287


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>