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

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 <[email protected]>
Cc: Paulo Zanoni <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]