drm/i915: Move cdclk and pll setup to intel_modeset_compute_config(), v2.

It makes more sense there, since these are computation steps that can
fail.

Changes since v1:
- Rename __intel_set_mode_checks to intel_modeset_checks (Matt Roper)
- Move intel_modeset_checks to before check_planes, so it won't
  have to be moved later.

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>