drm/i915: Remove dereferences of crtc->config in set_pipeconf/misc functions, v2.

One more user of crtc->config down. :)

Changes since v1:
- Constify crtc_state
- int pipe -> enum pipe pipe
- Move i9xx_set_pipeconf declaration to the other pipeconf declarations.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004094604.2646-2-maarten.lankhorst@linux.intel.com