drm/i915: Use local pipe_config varariable when available

In functions that define a local pipe_config variable to point to
crtc->config, replace remaining references to crtc->config with
the local variable. This makes the code more consistent and easier
to change in an automated manner.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>