drm/i915/ddi: s/crtc->config/old_crtc_state in haswell_crtc_disable()

crtc->config points to the old crtc state at the point
display.crtc_disable() is called, so use the more descriptive pointer
instead.

v2:
- Convert one remaining instance of the ptr in the function. (Ville)

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180613170710.15080-2-imre.deak@intel.com