drm/i915: Do not acquire crtc state to check clock during modeset, v4.

Parallel modesets are still not allowed, but this will allow updating
a different crtc during a modeset if the clock is not changed.

Additionally when all pipes are DPMS off the cdclk will be lowered
to the minimum allowed.

Changes since v1:
- Add dev_priv->active_crtcs for tracking which crtcs are active.
- Rename min_cdclk to min_pixclk and move to dev_priv.
- Add a active_crtcs mask which is updated atomically.
- Add intel_atomic_state->modeset which is set on modesets.
- Commit new pixclk/active_crtcs right after state swap.
Changes since v2:
- Make the changes related to max_pixel_rate calculations more readable.
Changes since v3:
- Add cherryview and missing WARN_ON to readout.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>