drm/i915: Do not run most checks when there's no modeset.

All the checks in intel_modeset_checks are only useful when a modeset
occurs, because there is nothing to update otherwise.

Same for power/cdclk changes, if there is no modeset they are noops.

Unfortunately intel_modeset_pipe_config still gets called without
modeset, because atomic hw readout isn't done yet.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by(IVB): Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>