drm/i915: Support modeset across multiple pipes

Compute new pipe_configs for all crtcs in the atomic state. The commit
part of the mode set (__intel_set_mode()) is already enabled to support
multiple pipes, the only thing missing was calculating a new pipe_config
for every crtc.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>