drm/i915: update pipe size at set_config time

This only affects the fastboot path as-is.  In that case, we simply need
to make sure that we update the pipe size at the first mode set.  Rather
than putting it off until we decide to flip (if indeed we do end up
flipping), update the pipe size as appropriate a bit earlier in the
set_config call.

This sets us up for better pipe tracking in later patches.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>