drm/i915: Use global atomic state for staged pll, config, v3.

Now that we can subclass drm_atomic_state we can also use it to keep
track of all the pll settings. atomic_state is a better place to hold
all shared state than keeping pll->new_config everywhere.

Changes since v1:
- Assert connection_mutex is held.
Changes since v2:
- Fix swapped arguments to kzalloc for intel_atomic_state_alloc. (Jani Nikula)

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>