drm/i915: factor out compute_config from __intel_set_mode v3

This allows us to calculate the full pipe config before we do any mode
setting work.

v2:
  - clarify comments about global vs. per-crtc mode set (Ander)
  - clean up unnecessary pipe_config = NULL setting (Ander)
v3:
  - fix pipe_config handling (alloc in compute_config, free in set_mode) (Jesse)
  - fix arg order in set_mode (Jesse)
  - fix failure path of set_config (Ander)

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>