drm/i915/atomic: convert intel_atomic.c to struct intel_display

Going forward, struct intel_display is the main display device data
pointer. Convert intel_atomic.c to struct intel_display.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/7ef6fe795e4e5c26ae0d546e57f64f494aaf56fc.1742554320.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>